^^^^
Actually, I have a fair bit of experience at this. Even when one is quite fit, it's not too difficult to duplicate the OP's problem. Just go out and ride a mountainous 400k, with say 20,000' of climbing, as hard as you can, and see how your legs feel the next day. And the day after that. Unless of course you've just completed a full brevet series, in which case it might take the 508 to put the hurt on you. That, on a smaller scale, is what the OP has done.
Mileage does equal base. I would even go further and say mileage = strength. You want to get really strong, ride 20,000 miles/year. That's the reason coaches recommend a certain number of base miles before doing long intense rides. During the season it's easy to lose base by doing rides at the limit of one's ability every weekend. One works up to such rides by the normal training periodization and workout progression. But when one starts doing extreme rides every weekend for a period of months, it can be difficult to recover between these hard rides, and one's base and thus fitness falls off. Hence the suggestion that Carmichael and I make.
The OP has not by any measure overtrained. His legs are just sore. It's no big deal. If he can manage to increase his midweek mileage, they will over a period of weeks become less sensitive to these hard rides. I find about 200 miles/week to be about right for doing 70 mile/6000' rides at one's limit every weekend. Whatever workout zones one can manage between these rides, without overtraining, are the correct zones to use. Their intensity will depend on the age, fitness, and experience of the rider.
In more general terms, the HR zones experienced in one's workout should form a pyramid, with the most hours spent riding easy and the fewest riding hard. This is at least partly due to the fact that each HR zone corresponds to certain energy producing chemical processes, and it's necessary to exercise all our chemical plants. We spend more time riding easy because we want as much energy as possible to be produced by the processes exercised in that zone, i.e. aerobic and fat burning. That's the principle of specificity. Those processes do the least damage and have the greatest endurance. And etc., on up to anaerobic processes which require high glycogen expenditures and are likely to do the most muscle damage. A downloading HRM is very helpful for measuring time-in-zone and thus planning future training.
The OP didn't do much easy riding on those hard rides. Therefore his fitness in the lower zones likely suffered, or at the least, did not improve. This alone is a good reason to use low intensity rides to recover and build fitness.
Recovery rides are very well known tools and are recommended by most respectable coaches. Nothing new here. Recovery rides do a lot more than you think, some of which I've outlined above. They loosen up the muscles and help the legs become supple again. They open the blood vessels and allow nutrients to more quickly and completely reach damaged tissue. Among these nutrients will be the nutrients necessary to rebuild muscle glycogen.
When the rider's legs are recovering, active recovery ensures a quicker and more complete recovery than does inactive recovery. Plus it does build cardiovascular and muscular conditioning. Even following extreme efforts such as the OP's, Lance's, or the brevet rider's, active recovery works the best and the fastest, and will return the rider to an even higher state of fitness. Believe me, I've had a lot of practice at it. If you haven't tried it, you should do so before inveighing against it.
TdF riders do ride on a rest day, you know.
All the above aside, one can race crits doing nothing but HIIT. Principle of specificity again, since that's what crits are. But the OP is not a trained racer, doesn't have the base of a racer, and is riding 2-3 times as long as the average crit. He needs different training.