what do I need to do and when?

What are you ulimate aims? if it is just to be able to ride 100 miles, well intervals really are not needed. Intervals are used to try and raise the power you can ride very hard at, if you are more interested in just cycling forget about them. Just the act of cycling more frequently and longer will help you improve your endurance for a novice.

Nutrition, there is no magic stuff to eat, just eat a healthy diet 90%+ of the time, it is OK to have the occasional treat. As you ride longer, you will need more fuel, but this can be gained from breakfast type bars, chocolate bars, fig rolls, malt loaf, jelly babies etc. There is no real need for sports bars, though they might be more conveinent

You might find as you get closer to the 100 miles, you might not be able to increase the mileage by 20% each time, but if you get to 70-80 miles doing 100 miles isn't much more difficult to be honest. As you get even more fitter, you will probably end up doing the longer rides faster, and possibly on less fuel as your body gets better at processing fat as a fuel.

Sounds like your doing the right thing at present by increasing your ride distances. Your speed should go up with your increase in fitness but it does depend on how hard your pushing during your training rides.

As for food you have lots of options and often the most simple are the easiest, personally my fav is malt loaf - but you just need to make sure you are replacing what you take out with good simple healthy foods.

You could always join a local club who will offer group rides at different speeds and probably different lengths. You'll meet fellow riders and learn about group riding plus most club runs on the weekends include a cafe stop too.

As you reach longer distances then you'll probably end up having to cut back on your other rides. If you're doing all these rides PLUS 3 days commuting then its going to quickly get tiring and the fatigue will start to build, and you'll risk burning out if you try and just ride long rides all the time!

You have several options really, but perhaps just stick to 1 long ride per week (the longest distance you are pushing yourself to do each week) and then keep the others shorter - perhaps going a little harder on a 25 mile loop each week, or maybe deliberately heading out on a shorter but hillier route. Every so often mix it up a little and do a couple of medium distance routes of 30-40 miles or so if you're feeling OK.

Just by getting out and riding regularly you will see constant benefits. Its takes time for your body to adapt to riding, and the first couple of years will all be about you simply getting better at riding by riding. All the rides will contribute to building your base fitness as long as you arent just rolling along at 10mph anyway, so get out as much as you can, rest plenty as you start to build the miles as you get fitter when you rest and not when you ride, and enjoy it!

Clubs are great if you like the social side of it or for motivating you when you might otherwise stay in or skip off early, as well as for learning group riding skills if you have a well organised club ride. The downside is that group riding where you spend a big % of your time riding out of the wind in a pack, really doesnt make for great training time. It will help you get used to being in the saddle for long periods, but you'll end up cruising along at a pretty easy pace for much of the time and the fitness benefits are greatly diminished. They have a role to play and I enjoy my club rides hugely, but for training I really ride solo.