Can I ask why you wish to merge them? If to obtain a track or similar, convert to gpx and there are easy ways to merge, notepad probably being the most basic. But probably not the easiest.
It's so that I can generate a map showing the whole of the route we took around Iceland. I've got 12 separate .fit files, one for each day, and I'd like to generate a single file for the whole trip.
Can I ask why you wish to merge them? If to obtain a track or similar, convert to gpx and there are easy ways to merge, notepad probably being the most basic. But probably not the easiest.