Pushing 40 here. While it takes me a good 3-5 days to recover, I like to do back to back days of hard efforts. Usually an interval or fartlek day followed by a tempo or threshold run.
My high school coach often cited a study that concluded that the effects of a hard workout are felt two days after said workout. The implication was that you could run back to back hard days. Couldn't tell you the name of the study, but my coach was very successful and well respected.
Under that assumption I've always done two hard days in a row, typically followed by an easy day, easy/moderate day, and long run. For example:
Mon: Easy run with strides and core
Tue: Easy run or off
Wed: intervals and core
Th: tempo or threshold
Fri: easy run with strides
Sat: easy or moderate run with pickups if feeling it, core
Sun: long run
Two things that may allow this to work. First, I don't race my interval days. Jogging the rest period helps keep me in check and I usually do one or two intervals less than I could if I pushed myself harder.
Second, my tempo/threshold runs are 5-6 miles and run closer to half marathon pace. A hard effort, but not "going to the well" line some might to run a true 20 min tempo.
I consider this all a modified, old guy way of training but it keeps me healthy and winning my age group.