Originally Posted by NoRacer
Going hard up the hills all of the time will get you "overtrained" or at least push you into a catabolism. After many sessions of high intensity hill climbs, you won't be stronger, you'll be weaker because you may start to break down your muscle tissue. You feel like you are getting better--likely your body is just becoming more tolerant to the lactic acid.
To get better, you need to spend long sessions in the saddle building an aerobic base. You refrain from reaching above your lactate threshold, because this will only stifle or damage the aerobic base.
If you want to race, then worry about high intensity sessions AFTER months of establishing an aerobic base.
+1.
High intensity training gives you headroom above your aerobic base. It doesn't increase your aerobic base, and as NoRacer notes, it can damage your aerobic base.
Intervals are to be approach with caution. Note that the interval isn't what makes you better, you get better on the recovery days.