Step 1: Go somewhere on your bike. Track it with the garmin.
Seems that garmin records it in a proprietary, binary FIT format? You can convert it to GPX using gpsbabel and to geojson also using gpsbabel, which is nifty.
Your map page can render geojsons natively, but you should only do that for rides which don't reveal your house.... You should strucutre the directories so that these raw gpx files arent kept in the part that gets served, even if you are showing their summary statistics
I guess you could have a page-per-travel, and a json-per-travel and a summary-per-travel all with the same name but different extensions, and then a big ol' script that converts the summary into a page and includes the json (ie, map route) where it is supplied, which is not all of the time... hmmm... yes....
When you make ur self-hosted php strava, you will need gpsbabel binary. Then you will need scripts that can do the maths using these files, and then you can party!