When I started I had trouble with a 4 mile ride around the lake, about a year and a half of regular 3-5 times a week riding and I ended up at a century. I ratcheted up the mileage but never worried much about speed and still could care less about it. Dropped riding and replaced it with running and did a half marathon last year but now I'm back to riding as it's a whole lot easier on me and I just enjoy it a lot more. My best advice, just ride the bike.