Here is a rough guideline to measure if your current goals are realistic
THREE HOURS AND UNDER – If you have done any of the following: a marathon under 3 hours and ten minutes, 5k under 18 minutes, 10k under 38 minutes, or a half marathon under 1 hour 24 minutes
FOUR HOURS AND UNDER – If you have done any of the following: a marathon under 4 hours and ten minutes, 5k under 24 minutes, 10k under 50 minutes, or a half marathon under 1 hour and 50 minutes
FIVE HOURS AND UNDER – If you have done any of the following: a marathon under 5 hours and 10 minutes, 5k under 29 minutes, 10k under 1 hour and 3 minutes, or a half marathon under 2 hours and 20 minutes
FIVE HOURS AND OVER – If you haven’t run any races to get a gauge on your time, or if you haven’t broken 5 hours yet, then it is better not to have any specific time in mind.