Is Short and Sweet challenge enough

I've been doing FB for a few years now and just recently joined FB Plus. I decided to try out the Short and Sweet Challenge because things are kind of busy right now. It has worked out well and I am on track to finish it Friday. I really have enjoyed the short workouts, but are they enough? I guess to get opinions on that question, I need to give a little more details. I run about 2 miles Tuesdays, Thursdays, and Saturdays. I enjoy the running, but I actually need to keep it up because I firmly believe it is what keeps me off diabetes medicine., so my runs are non-negotiable. However, it seems like when I try to do a program (loved Strong), I wind up getting shin splints and I think it's just from overtraining. I also haven't been able to maintain a good routine because I have added a few days a week working. But the summer is coming up and I woud have more free time, so my mind says "Ok, time to really kick in the working out" Problem is, I may not really have as much free time as I am thinking. So, I was wondering if I kept up the short daily workouts if that would be enough to actually tone up? My thoughts are to do one of these

-short workouts (15-20 min) daily Mon-Fri

-30-40 min workouts 3 days a week

Personally, I feel like the 3 days a week would be better, but sometimes I just have really busy days and if I have a longer workout scheduled, I will just skip (and yes I know I can always just find a quick workout to do, but I am being honest about myself here and know that I won't do that). Whereas, since I've been doing the short challenge, I find myself saying, it's only 15 minutes, you can work it in. I also know I can make the short ones harder by lifting heavier, it's just a mind thing for me that 15 minutes isn't enough. Anyway any advice?