There are 4 variables.
1: How hard you work
2. Your ability to train consistently and avoid injury
3. Your genetic trainability (how much you improve with X# of training sessions)
4. Your physical maturity at present, and how your body matures into your senior year
You can only control variable #1 and #2. So work hard and stay on top of health issues.
#3 will reveal itself in time. Are you a trainability monster (ala Jim Ryun) who can absorb incredible beatings in training only to emerge faster? Or are you a naturally fast freshman who will only improve in small increments despite hard work? Read the Sports Gene if you are interested in understanding this phenomenon.
#4 Will also reveal itself, though you already may have a clue. If you are a tiny little dude for a freshman with no leg hair and are already running close to 5 flat, you will likely drop chunks of time effortlessly as the extra testosterone of puberty kicks in. If you are one of those guys who is already pretty tall, has leg hair, and has hit your big growth spurt, time improvements will be harder to come by.
I was a physically mature freshman with low/medium trainability. Despite working hard, I didn't improve as much as some of my teammates who were on the same training program (or some who ran less than I did).
Worry about #1 and #2. #3 and #4 will reveal itself in time. If i could go back in time I would have worked hard, stayed on top of aches and pains, and worried a hell of a lot less if I was a failure for reaching a given time or not.