You have that British guy, Steve Way(?) racing 100km and setting a national record a couple of weeks after London. Another Brit from the old days, Cavin Woodward used to do multiple marathons usually around 2:20, or ultras, in a month or so. Doug Kurtis and Kjell-Erik Stahl ran sub 2:20 marathons monthly at times. I once got a mile PR at an indoor meet the night after setting a marathon PR and then came back with what was then a passably decent two mile.
The key is to know yourself and your circumstances. NotAustin thinks that because Meb isn't running hard yet that no one else who ran Boston should either. But Hall ran 2:17, not 2:08 and even if he's not the runner he was 4-5 years ago it's unlikely that 2:17 was anywhere near as intense an effort for him as the 2:08 was for Meb and while Hall is not Rupp, a 30:36 10,000 meter race is likely nowhere near an all out run for him. Taken to a full marathon that pace would give him around 2:09 or so, probably slower than he's hoping he can be eventually.
Is he risking injury? Heck, you risk injury every time you go for a run but there was nothing outlandish about what Hall did.