Almost zero-L2P, advice required

You don't give an awful lot of information about your current fitness levels, whether you are still looking to lose weight etc. but I would say in your position, doing any kind of interval training is completely pointless. It might increase your peak power output, but it's going to do very little for your endurance over multiple hours in the saddle.

If you were looking into racing crits then intervals might be useful, but the L2P is essentially back-to-back long group rides, so that's what you should be focusing your training on. The commute is good to keep things ticking over in the week but I'm guessing your weekend rides of 1-3 hours aren't getting close to 100 km at the moment, so you should be focusing on getting regular (i.e. weekly) rides up to and over that distance before the trip.

If that doesn't convince you, then have a read of these articles. You don't need a power meter to apply what they're saying, essentially that slow steady riding has many physiological benefits, but intervals a few very specific ones.

http://home.trainingpeaks.com/blog/arti ... e-athletes
http://home.trainingpeaks.com/blog/arti ... ing-levels

And try not to over-think things, just concentrate on getting those regular commuting miles in as often as possible, try to make time for as long a ride as possible at the weekend and take rest when you need it to avoid injury. Good luck!

Luckily although it might seem it at first, depending on the pace and terrain doing 100k in a day isn't really that hard, but endurance is the key.

Intervals are useful for developing your power which will help on hills and headwinds- as you become a stronger rider you will find climbing at a leisurely pace much easier and so wont be suffering as much by the end of the 100k.
However, you still need the endurance to be able to keep going- for this being able to do as many miles as you can will help- such as commuting every day if you can, perhaps adding on some extra miles on the way home a couple of times per week if you feel like it.

Then on the weekend where it suggests the steady miles aim to increase the distance/ time on the bike in small increments until you are beating 100k reliably in these rides, even if it takes a while. However, being able to complete 100k within 5 hours is a good target to set yourself some weeks before the big day. Joining a local club on their club run will help here- most go for distances around 50miles each week, which is about 80k by itself. Add on a small loop on the way home and you'll have hit the target!

Good luck!