Look on the website of the prospective race and find the “course closing time” (CCT). This is the slowest time allowed as an “official finisher.” Look at your recent races and/or your recent “Magic Miles” (read all of the details see here). It’s not a good idea to enter an event when your predicted finish time or recent finish times are close to or slower than CCT).
What is the enrollment? Large races are often more fun and engaging, with crowds of spectators. The downside is the weaving between runners can slow you down. Each time you zig or zag, you’ll add to the distance covered. In races with over 30,000 participants, the extra distance is often a mile or more in a marathon/half a mile in a half marathon. Extra distance means a slower finish time.
What is the projected weather? Temperature has a major effect on finish time. I’ve surveyed this and found that most marathoners slow down 30 seconds a mile for every 5 degree increase above 60F. This means that metric/centigrade runners will slow down 20 sec per kilometer for every 2 degree temperature increase above 14C.
Sightseeing? I suggest investigating points of interest in the area: museums, national parks, etc. The Disney events, for example, offer activities for family members and reward them for enduring your absence during long runs.
Terrain, elevation change, and past participants’ comments.
Visit www.marathonguide.com for lots of good information.