FWIW, this thought also went through my mind - just not enough info to say yay or nay. I think leg strength is important to stay healthy running, but there's still that line - are your legs too stressed combining HIIT with increasing mileage.
If it helps, when I'm going through higher volume periods wrt mileage I keep the leg strength exercises lower impact. The kb swings gyno mentioned, single leg rdl's, walking lunges, squat-and-throw's, russians, box jumps, etc. They're meant to compliment the running; not supplement. I will switch it up and do higher demanding strength training during low volume mileage periods.
Since you're in the mileage building phase I would be sure to keep leg strength low impact. If you're going to push yourself on strength days just do it from the waist up.