This is the best advice you've received.
You complain about having sore legs after 5 miles, which says to me that you are trying to cycle using strength, rather than endurance. You should be sitting down the whole time, and spinning smaller gears. No one push of the pedals should be hard work. You have the choice of low revolutions/ high load against high revolutions/ low load. Millions of cyclists make the mistake of going for the low revs/ high load option, and then can't do any great distance, or climb hills.
Think of it this way: look at 100m sprinters' physiques. They are huge, muscly creatures with over-developed thighs. Have you ever seen one of them try to run more than 400 metres? It isn't pretty..... Then look at Mo Farrah. How far behind would he be in a race over 100m with the proper sprinters? Miles... Yet how far ahead would he be in a race over a mile? Cycling is a long-distance event (with the odd exception of some indoor track events). You simply cannot get enough oxygen to big thighs to keep them doing all the power work for, say, a 50 mile ride. You need to think of yourself as Mo Farrah on a bike, not Linford Christie. Mo's legs look like they'd break if he got into too high a gear and tried to push with all his strength, so what would he have to do to be a good cyclist? He'd have to spin those pedals at high revs/ low load, and use his endurance.
Mike