When I started running my freshman year of high school (track only, no XC), I had never run before and I had only done one sport: swimming, the previous winter season. I was equally slow at everything so they just put me in distance for the hell of it. I think my first 3200 was 17:18 (so 8:39 pace, which is significantly slower than my current easy run pace). When I ran XC the following fall (my sophomore year) I was definitely one of the slowest on the team. My debut 5K was 30:33 and I walked at least half a mile of it. I dropped my time down to 26:22. Unfortunately I got injured before the conference meet and ended up crawling home in 31:19 (my all-time slowest 5K). After track that year, I started really buying into running and I started dropping time left and right. Here's my progression:
Freshman track: 3:09 800, 17:02 3200
Sophomore XC: 26:22 5K
Sophomore track: 3:01 800, 6:50 1600, 15:55 3200
Junior XC: 22:17 5K
Junior track: 2:40 800, 5:51 1600, 12:46 3200
Senior XC: 20:22 5K
Senior track: 2:40 800 5:49 1600 12:27 3200
Freshman college XC: 24:44 6K
Freshman college track: 5:49 mile 11:33 3K 20:21 5K 12:39 steeple
Sophomore year XC: 23:28 6K
Sophomore year track: 5:34 mile 10:54 3K 19:29 5K
Junior year XC: 23:21 6K
Junior year track: 5:47 mile 10:56 3K 18:25 5K 11:38 steeple
Now I'm going into my senior year with big goals.
TL;DR: Just go for it. Don't worry about how slow or fast you are because you'll inevitably improve, maybe even a lot. So far I've dropped my 5K time by 42% (or 12 minutes and 54 seconds).