.Fit files are also east to use. If those are what it uses already am I missing the export function or us it just an internal thing? I've read the threads on exporting data from webpages and have done it, but it is tedious. I suspect I could automate something, but haven't ever hit round to it.
As well as being fine to use I believe there are easy ways to combine .fit files and convert to TCX.
My understanding of TCX is that they are just a sub-type of eXtensible Markup Language (XML) files. Suspect it would be childs' play for you to export it, but I'm not sure I see much logic in you doing so if you already have .fit files.
If you do decide to export .fit files Training Peaks allow import of those files and I'll suggest to them they allow importing of your metrics. I know they're working on adding Garmin Running Dynamics (may already be there). TP also does cycling, running, gym etc. so it is quite convenient. I think it may even have a built in file combining ability, although I haven't tried it.