In reply to Chay:
I've not read all of the advice here, so may be repeating what others have said - but I think I come fro a reasonable position of knowledge.
Firstly, you aren't taking on enough energy during your 80 mile rides. You should eat a decent meal (porridge etc) 3 hours before you start. The 3 hours is key to allow your body the time to process food into energy. You should then be drinking a minimum of 2 750ml bottles with energy drink diluted as per instructions and stuff your pockets with bananas/flapjack/energy bars. 80 miles is a long way for any cyclist - I ride a 25 mile tt in under 55 minutes, so am fairly bike fit, but I rarely ride 80 miles, and when I do, i'm tired!
Secondly, you either need to do high intensity intervals, which will increase the power that you can produce for a given heart rate, or allow you to work at a lower heart rate whilst producing the same power, thus enabling you to be fresher at the end of a long ride.
Or
you need to regularly ride for longer that 90 minutes. 90 minutes is key because it is the amount of time that you can using energy stored within your liver, blood and muscles. This enables your body to start burning fat more efficiently as fuel. Even the skinniest cyclsist has enough fat to fuel days on continuous exercise, our glycogen stores are limited to 90 minutes. If you are doing these 'fat burning' rides, then take less food with you. These will hurt in a different way, because you are forcing your body to burn something that it isn't used to. Imaging running a petrol car on diesel!
There is no easy way to get fit to be fresh at the end of an 80 mile ride. Personally I don't get your insistence to do high intensity work in the gym. It sounds like you have reasonable fitness, but not outstanding fitness, so you should be focused on marginal gains, you should be focused on things that will give you massive gains!! If you are time constrained, this means intervals - which suck!
for climbs, the best thing you can do is loose weight - I didn't catch whether this is an option for you, but worth considering an an alternative if you don't want to do intervals. cycling is mostly about power to weight ratio, if you can't increase power, decrease weight.