I just got back into cycling recently and my lower thighs and the outside of my knees have been on fire (lactic acid, not actual pain) if I pedal for more than 45 seconds. Cadence is irrelevant, I get the same burn at 110rpm in a low gear as at 80rpm in an intermediate gear, though it manifests nearly twice as fast at 110rpm. It's always localized to the very bottom of my thighs - right above the knee cap, running across the whole width of the thigh, as well as the outside of my knees along the skin (nothing deep in the knee). Saddle height doesn't seem to have an effect either. The only reason I think it might be a fit issue is because the majority of my thighs are fine and my breathing is steady.

Or is this normal for a beginner and I need to HTFU?