This depends on so many things.
If you did 15 mile long run and it was close to max effort for your long runs (think last 8 at half marathon goal pace) and your interval workout was a long broken tempo workout (10x mile at half marathon pace) 12x1k at 10k pace 5xmile at 5k pace. Then no that was not optimal. But did it ruin your long run gains?! Probably not it probably just ruined the next 5 days of running but in the long run is just an unnecessary injury risk.
Now if you ran 15 miles the majority easy or heck even medium and did one of the above style workouts you are completely fine with not huge injury risk.
If you intervals the next day was something like 300s or 200s to focus on leg speed and form you are completely fine.
I would argue it's probably more optimal to swap intervals with a long run unless the long run is completely easy. For instance in marathon training I'll do a huge long workout 10-15 miles of work, then an easy 16-20 the next day and then a shorter leg speed day the next. That 16-20 is a long run but it's all easy. I am training time on feet and running efficiency not like a hard long run where you are training running fast on tired legs. Context of what you are training for is very important too and context of your milage the workout the previous weeks etc.