Having started biking very late into the season (about a month ago), I've been doing laps around a local rail trail (~6 miles per lap) to build stamina and muscle in preparation for a ~36 mile ride to be done sometime in August. I have a riding partner whom I meet with at least every other night to do some riding.
I'm a bigger fellow, been hovering around the 250lb mark since high school (I'm 26 now). My riding partner is the polar opposite, a slightly shorter, very skinny fellow who weighs maybe 135. Despite being much lighter and more nimble on climbs and technical (off road) trails, he lacks stamina to keep up with me on longer stints. I have a feeling this has much to do with his bike, he's riding around on a fairly neglected wal mart mongoose dual suspension mountain bike, which has very high rolling resistance (probably from lack of lubrication somewhere or other). On a slight downgrade, I can coast past him while he's still pedalling. I think I've talked him into upgrading to a new low-to-mid range hard tail mountain bike (~$500 is his limit) which should vastly reduce the amount of energy he's wasting on fighting against the bike. We'll find out after Saturday, which is when I'm going bike shopping with him. But I digress.
After a month of steady riding, doing between one and three laps a day (6 to 18 miles), 3-4 days a week, I realized I'm actually getting much faster. When we first started out for the season (last season we only biked a few times so it's actually been at least a year and a half since we've been in biking shape), we both struggled to finish one lap in 45 minutes. Last night we finished the last lap in 25 minutes. I just put smooth tires on my bike ('07 cannondale f4) since I've realized I rarely go on terrain rougher than fine gravel, and I gained an extra mph or two from that alone vs knobbies. The cyclocomputer I have said I averaged ~14mph over the last lap.
This pace was still fairly exhausting, after two laps I was done for the evening. I have a feeling if I slowed down a bit, I'd have no problem doing the 36 mile trip. Guess we'll find out in a couple weeks...
As for why this season is wonderful, the heat wave is absolutely stunning. This is one of the major reasons we've been riding late at night (after 10PM, sometimes after midnight). Anyone else do night riding regularly to avoid this heat? Today the heat index is 110F! At night it usually drops at least 15 degrees, sometimes 20. Far more comfortable, plus there's nobody around to gawk at the fatty with his shirt off!