Quote Originally Posted by Nokomis
Once I hit that, my quads feel like lead...
This may or may not have anything to do with it, but you don't mention what your pedal setup is. If you're not clipped/caged/or somehow 'attached' to your pedal, then your quads are possibly having to work too hard. Since I switched from loose toe cages to PowerGrips, I've been amazed at how much more my hamstrings and glutes work in difficult pedaling situations and I haven't felt any soreness in my quads since. If you've already got clipless pedals, though, I'm not sure what to suggest.