Effort causes aching legs. How much effort depends on your strength and conditioning. If I went in and squatted a few sets with 200 lbs., I'd have aching legs before I was done, but a powerlifter would do that for warmup. We did 20 miles on our tandem yesterday evening, including a few full power intervals, and yup, I have aching legs today. But I wanted a few microtears.

In your case, it's just scaled down. 1/2 mile hurts. Try starting out much easier, so that it feels effortless, the pedals feeling light. After about 10 minutes of that, gradually increase force on the pedals. I'm not surprised that the second lap was easier than the first. I'd keep doing just that, but try to do it every day, and try to do the first lap with so little effort that it doesn't hurt. Now it may be that just moving your legs in circles is a considerable effort. I know that one legged pedaling on rollers, even in a tiny gear, will begin to hurt after a few minutes, so I sympathize. But try to keep the warmup effort down, anyway.

And eating what you did is a good plan. If you want a fun check, tape your belly and thigh every few days. You want the thigh measurement to go up, the belly not so much.