I would not run a half marathon if u haven't been training the past few weeks, prego or not. Walking it would be fine if u are fit enough. If u've not been doing much the past few weeks, I would suggest easing yourself back into running and staying at a level that u feel comfortable with. Just what I would do... I'm also a fitness gal and have worked out and ran 5 days/ week so fat at 21 weeks :)
After I got over being sick I started up jogging again and my normal routine beside abs you'll know when it's to much for you then just take break but part of a healthy pregnancy is staying active :-)
I exercised alot before I was pregnant and when I exercised pregnant I got the wind knocked out of me. I suggest if you dont feel that feeling of the wind being knocked out of u than try a jog, but if you are feeling light heart drink water an walk.