@Stonechat its more difficult with fit files, I use two utilities one called
tcx convertor which can export a plotted ride as a history file, and
firstobject xml editor
if you have the raw tcx file the important bits are between the first and end <lap> markers, there may only be a pair it depends how you have your unit set up yo record laps, mine id every 5km, You also need the <Id> tag line this identifies the activity, This is from the header of a tcx file, the red bit is from the tcx converter and is removed the green bit is the activity identifier, the yellow is the rides total/average details (for some reason though the 705 sometimes stops recording the gps track, it still records the ride, even with out a sp sensor), so its not like I put it on pause ect...
Below the yellow section is the first track point,
A tcx file direct from the 705, the yellow is the lap totals as in the one above but that lap is for the total (something that was lost), and the red the first track point of that lap, you can see the difference in the header.
This is why I can reset my garmin say at a friends, then I can stitch the files together, copy header, to the last lap marker in the first file, and then and then from the first lap marker, under the id tag, (if this was included, strava would load it as two separate activities), and then past that at the end of the original (or first part)
This is the footer, the end lap point as I mentioned, the green bit is the bit you remove from the first file if you stitch things together, but you keep it from the file you copy
Its a while since I did a fit repair, you need to export the fit file you have providing its not corrupted as a tcx file, say with rwgps (don't correct data) delete the footer, than using rwgps, plot the missing bit, export as course, load that to tcx converter, make sure you adjust the avg speed to what your average speed over that course was (it will effect your p.b's on any strava segment), export that as a history file, and then join the two as described.
And that is probably as clear as mud, only one cup so far.