Steve would be the expert here ... but ...
First of all doing "too many reps" does not create lactic acid. Lactic acid accumulates with increased intensity - when you hit an anaerobic training level, then lactic acids begin to accumulate in the muscles. If you're weight lifting, you might reach that point with lifting really heavy or with doing a lot of reps ... but "too many" has no meaning in this instance.
My understanding (and perhaps Steve can give more complete information) is that lactic acid does become an energy source at some point - and some studies have shown that it's responsible for the "2nd wind" type of recovery that distance athletes experience during a performance.
But people who are losing weight and who are not concerned with detailed performance issues need to break away from this whole "you burn X (lactic acid, glucose, whatever) instead of fat" mindset. Because when you're talking about average, normal, every day weight loss, it doesn't matter.
Ultimately, it's about how many calories you burn vs. how many calories you take in. You will lose fat if you eat at a calorie deficit. You will lose more fat and less muscle if you eat at a calorie deficit and work the muscles with enough intensity to maintain muscle mass.
Everyone wants to take it down to this molecular level and burn only fat ... and it just doesn't work that way. You can't make yourself crazy with pseudo-science like that.
