New cycling addict here, a 40-year-old "clydesdale" weighing in at 235 pounds (I've lost 5 pounds over the last month or so). In the past 12 weeks since winter broke I've seen my average weekly speed go up from 18 km/h to my current 24 km/h. I can feel my strength and stamina improving weekly, which is exciting. But here's my current problem, which I was wondering how some of you might address if so inclined:

Where 12 weeks ago a hill would leave me out of breath, and I would feel exhausted after rides, I now no longer do. I'm now going faster up hills, and rides of 40 and 50 kms are a walk in the park. This hopping off the bike after an hour and a half and not feeling the least bit of fatigue has been a revelation for me, so I want to increase my average speed, but I've got a bottleneck: the muscles just above my knees.

What do you do when the heart and lungs are raring to go, but the legs can't match them in performance? Do I just have to be patient, and wait for my legs to get stronger? I'm certainly hoping to slim down a whole bunch this year--it doesn't take a brain surgeon to see that losing another 35 pounds or more would do wonders for my legs. Still, I wonder if anyone has had a similar experience and might have some pointers to offer?