Hey dude,
I can understand your dilemma well. I was around your speed in high school although I had not yet considered the possibility of joining the military.
Instead, following high school I went to a local community college and eventually was picked up to run for a few years at a D1 school. Upon graduation I went to OCS and now fly Cobras in the Marines.
Looking back I sometimes wish I had done something along the lines of SF or SEALS. However, due to the long contracts involved with aviation, by the time I had the desire to attempt SOF it was no longer an option due to my age. The route I took and what you are proposing have positives and negatives. But in the end I think you'll be happy with what ever you do.
One thing I'd keep in mind and realize is that your prep for SFAS and the Q course and that of a high school track athlete are pretty far off. I'd recommend you make a decision and give it your all preping for either.
I have to get going, but if you have any questions about what I'd do differently advice on training for SFAS, etc let me know. Happy to help out a young motivator.