My friend is similar, loves running and ran until I think around 24 weeks? She said she started to struggle then as her belly popped out and she felt heavier. After that she just walked.
As long as she pays attention to what her body is telling her she should be fine!
My dr said that as a rule they suggest your temp shouldn't get higher than 40 degrees because we can sweat to cool down but bubs can't. (So running in the hottest part of the day really isn't a good idea... Not that it is anyway! But YKWIM.)