Join Date: May 2007
Location: SF Bay Area
Bikes: Battaglin MAX, Chorus kit; Bianchi 928 Veloce; Ritchey Road Logic, 7410 Dura Ace
Both of those activities require you to go anaerobic. Try adding hill intervals to your training. The way I used to do it was using a couple 1-2 minute long climbs on my regular route. Ride easy to the base of them, then push hard to the crest. If you use an HRM, you can watch your HR shoot right up. Let it drop back down about 30-40 bpm, and repeat.
You WILL get winded doing this, and that's the point - you're developing your capacity to keep pushing when your oxygen needs exceed your ability to suck in air and deliver it to the tissues.
Sprints, the skinny guys should have no advantage over you. Climbs? They're lifting about 70-100 lbs less than you, so you have to put out WAY more energy to keep pace with them.