You know: You forgot to reset your ride on your Garmin and Strava won’t let you split rides (without contacting support).
Just use gpsbabel:
gpsbabel -t -i garmin_fit -f 2015-09-26-06-25-01.fit -x track,start=2015092716 -o gpx -F day2.gpx gpsbabel -t -i garmin_fit -f 2015-09-26-06-25-01.fit -x track,stop=2015092716 -o gpx -F day1.gpx
Start/stop times in GMT.
Actually, use -o gpx,garminextensions to ensure heart rate etc is written to GPX.