The best first goal is to walk for half an hour and feel fine at the end. The best next goal is to jog or run for half an hour and feel fine at the end. The best next goal is to jog or run for 3 miles (5K) as long as you do it at a comfortable pace in which you could converse with a running buddy. For your first 5K, forget time -- just focus on finishing the distance.
Setting a race goal like a 5K race is a very positive approach in achieving your desired fitness level. The distance is short plus it gives you an opportunity to assess your performance after the race. More importantly, it opens more challenges to tackle by you completing the race.