I faced similar problems, I was also diagnosed at 19 and was very active. Anyway I had my ICD put in last year and today I run 3 miles almost every day, lift weights (light but with tons of reps so I still get results). So this is a topic near and dear to my heart (no pun intended). Have you seen and HCM specialist? If not you must because the totality of your condition and family history need to be examined, none-the-less knowing you have HCM does put you at risk to just drop dead so I would think doing 6 miles + the gym on regular basis without an ICD is a very, very bad idea.
BTW you can also drop dead even while "at rest", anyone with HCM should not think that being a couch potato means sudden death is negated.
Please go see a big shot type doctor, BTW are you on any medications? What is being done to treat your HCM, anything?