I'm in my twenties and would not consider myself a serious runner. However, I enjoy running 5 and 10ks with my girlfriend, who is a talented runner. I can barely beat her, running 5ks in the mid 17s. I used her HR monitor the other day, and I can't get it over 173. Shouldn't my max HR be closer to 195? Is there a way to increase your capacity, or would that just come with running more miles?