I rode about 120 miles/week at 15/16 mph for a solid month and for the past two
days I was hoping to speed up to a constant pace of 16~17 mph for at least an hour and I just couldn't do it. I've ridden 15mph for 2/3 hours non-stop but when I tried to
bump it up to 17 mph for an hour, I simply could not.
Incremental progression. Take a smaller jump in intensity (speed) or volume (distance).
my biking
muscles just aren't building up.
As you get more advanced, playing the sport is not enough. That's where strength & conditioning comes in. From your words, it seems that you lack strength/mass. Do some heavy lifting: squat, bench, deadlift, press. Every movement can be expressed as a percentage of your strength. More strength = smaller percentage = less fatigue = more endurance.