My Weak Legs

I have been doing bike intervals for the past couple of months and I always find that my legs are the weakness. Each interval session has a HR target, but I find as though I am usually short of this target, not because my lungs can't handle it, but because my legs are on fire and I can't keep up the pace.

Today's goal of 6x4min @ 82-88% of MHR @ 70rpm with 3min easy spinning at 95rpm, was another example.

1: 144 avg HR/147 max HR/ 68rpm
2: 142/147/69
3: 142/147/70
4: 142/148/69
5: 145/149/70
6: 142/148/69

My legs just won't pump fast enough, strong enough, long enough to get my HR up higher.

Mileage: 14.79
Time/Avg Sp/Max Sp: 53:01/16.7/23.3
Avg HR/Max HR: 135/149