avoiding the bonk

"Blowing up", as you put it, and bonking may be different things altogether. You should expect to be tired at the end of an endurance ride. You may in fact be eating too much. I have always felt that a Powerbar is pretty hard to chew and digest and doesn't really satisfy on a training ride or race either.
My best results have been with "real food" like small cakey things (pikelets, scones, fruit bars etc.) I can't stand the formulated stuff. Last time I raced a long race with that sort of food (some kind of gel in sachets) I suffered massive cramps late in the race.

From your speed I would gather that you are new to this sort of distance. So you shouldn't really be feeling as fresh as a daisy!

I should also add that it takes a lot of time to build this sort of endurance so don't worry. Just stop wasting your money on Powerbars!

Quoted post said:

Originally posted by catch
On a long ride - how do you calculate how often and how much you should eat - I am now doing around a 100miles at 15miles/hr on the flat and eat about 200kcal every 20miles in the form of a power bar - I top this up with an electrolyte / carb drink (not high carb) but towards the end of the ride I still feel as though I am starting to blow up - I have never blown up completely but always feel it is close - Is this a normal feeling? Am I eating enough

By "close to blowing up", I'll assume you mean your HR gets higher late in the route and doesn't want to come down. Believe this is normal, at least it happens to me during a Century. And as tafi said, the answer is more training rides to build endurance.

Your calorie intake sounds OK. All those Powerbars isn't what I'd care to eat either, but everyone's different here. The little sandwich cracker packs, with cheese or peanut butter, work for me. Every quick stop here has them. They supply about 200-220 kcals of mostly carb, and are usually priced at 25 or 35 cents. I grab a couple with my bottled water or Gatorade and I'm all set.

Don't forget plenty of water also, at least a bottle an hour or more depending on the heat and humidity. When the heat index is really high, ie 100*F, and the sun is intense, I'll go through almost a liter an hour. I've seen a chart showing the rise in HR caused by dehydration and it's not a pretty picture.