Knackered after first 50 miler - where did I go wrong?

I did my first 50 miler last weekend, well 51.5 miles to be precise, and the last 5 or six miles I felt like death, so I just want to know where I went wrong so I know what to do next time. I did the route in just a bit under 4 hours, not including stoppage time.
Been gradually building up, my previous furthest distance was 44 miles, but this was about 7 weeks ago, then I had an accident which took me off the bike for a couple of weeks. Been building again, and got to 39 miles before I attempted the 50 last week.
Can't remember what I ate the night before, but I started the day with just a bowl of porridge. Took two 750 ml bottles of water (with a bit of Robinsons Orange added), and a couple of Eccles cakes to eat on the way round.
The route was 51.5 miles with approx 4,820 feet of ascent according to my Garmin 305.
Was the jump from 39 to 50 too much? Was it the amount of climbing involved? SportsTracks said the 44% of the route was ascent. Did I eat the wrong things before leaving, or during the ride?
Want to try another long ride in the near future, but don't want to feel exhausted like this time.
Advice appreciated
Been gradually building up, my previous furthest distance was 44 miles, but this was about 7 weeks ago, then I had an accident which took me off the bike for a couple of weeks. Been building again, and got to 39 miles before I attempted the 50 last week.
Can't remember what I ate the night before, but I started the day with just a bowl of porridge. Took two 750 ml bottles of water (with a bit of Robinsons Orange added), and a couple of Eccles cakes to eat on the way round.
The route was 51.5 miles with approx 4,820 feet of ascent according to my Garmin 305.
Was the jump from 39 to 50 too much? Was it the amount of climbing involved? SportsTracks said the 44% of the route was ascent. Did I eat the wrong things before leaving, or during the ride?
Want to try another long ride in the near future, but don't want to feel exhausted like this time.
Advice appreciated

Ribble New Sportive
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Two weeks off a bike is a long time in cycling terms and when you did get back you went from 39 miles to 51.
Keep at it and see how you feel after doing the loop on a regular basis, you will get better.
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It's mostly a psychological barrier rather then a physical you need to keep breaking. You can be too intense with the mileage and the average speed and this and that....just cycle the distance at an easy pace a few times, then try a speedier test...you'll soon find that 50 miles is a doodle.
I usually do 40-45 miles on a Sunday morning. Just after Easter I started training again (due to a broken arm - non bike related). 40 miles were a killer and I'd dramatically get home exhausted! Now, same route, much quicker I get home, shower and then straight to the supermarket without even the need for food or isotonic drinks to recover!
Keep at it, enjoy it without worrying. The real next big test is 100 miles and that's when it starts to get hard
Kona Jake the Snake
Merlin Malt 4
10m tt pb:23:42.
25m tt pb: 1h 2min( only done 2)
Anyway, will try having a bit more in the way of energy food next time, and see how I fare.
Thanks for the responses.
Specialized Pitch Pro
In addition to breakfast, munch on something before the start. Two Eccles cakes isn't enough, even though they are very nice!
2004 Marin Muirwoods Hybrid
The next time you go out for a ride, take a couple of Tracker bars (or similar ) with you. Eat it in stages, so try chewing a third or so at a time. It feels awkward at first, but once you're comfortable with it, you'll find your energy levels should be more consistent.
2004 Marin Muirwoods Hybrid
momentarily from the herd and thinks for himself.
It was much hillier than I expected to be honest, but then I really don't have an idea of what you would class as a flat ride over that kind of distance.
Specialized Pitch Pro
You could still walk into the house?
Didn't ride hard enough!!
Sounds pretty normal to me. No worries.
But it depends on your fitness - some people are better at taking energy from their stored fat (that takes time to build up such fitness) and others like me, prefer to eat really regularly.
You'll find a banana will hit your bloodstream in about 25 minutes; gels a bit quicker. The more fat in the food the longer it takes to give you a boost.
And you need to take regular sips from your bottles to help you digest the food as well as keep you hydrated.
Also, as previous posters point out, psychology plays a massive part here...
Yes, loads of places do. Evans should have some plain ones, or the Gore brand is good. You have loads of options! I think I saw someone suggesting Wiggle further up, so do look there, or even eBay - lots of bargains to be had!
Plus my wife nearly had a fit when she saw I'd spend 200 quid on top of the bike for shorts, shoes, pump, bottle cages, computer, spare inner tube and a few other bits and bobs. Telling her I got the pedals for free didn't seem to help somehow. If I spend another chunk of money on a top and a jacket she'll really go mental!
Maybe for Christmas as in-laws will understand buying me clothes (even if they're cycling specific) rather than my usual requests for books or car related things (tyres and an oil pressure gauge this year!)
Luckily I've got some base layers and panelled jackets from rowing. The following look like good value for a basic top that she won't laugh at me for wearing! (unless anyone thinks otherwise)
http://www.evanscycles.com/products/oakley/side-plate-short-sleeve-jersey-ec023334
http://www.evanscycles.com/products/scott/entry-2009-short-sleeve-jersey-ec019875
Thanks!
2004 Marin Muirwoods Hybrid
Are you sure this is correct? 4820 feet?
I cycled 51 miles around the mountains of Snowdonia this morning up two mountains with the usual ups and downs too. My gps thingy said I'd done just over 3000 feet of climbing. Snowdon itself is 3560 above sea level, that would mean that in just 50 miles you'd climbed the equivalent of thje height of snowdon plus another 1260 feet. That just doesn't sound right. the whole route would have to be uphill.
Sorry to be questioning you and if the 4820 figure is correct then that is definitely the reason why the last few miles were hard.
TBH I just looked at the evans ones from the cheapest up until I saw some that looked OK. If I wear it out (because it's cheap and not very good) then I have an excuse to persuade the mrs to let me get some more expensive / better made stuff.
Sounds like I need to find the nearest Aldi though...
2004 Marin Muirwoods Hybrid
Specialized Pitch Pro
A shortish 40 miler I normally do is over 2800ft on the GPS and that isn't even in a hill or mountain area. The ups and downs are accumulative and can be deceptive.
Kona Jake the Snake
Merlin Malt 4
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Some big reductions there! But only in odd sizes
Not that I'm calling you oddly sized!
2004 Marin Muirwoods Hybrid