Any Runkeeper users? Looking help with presenting & creating maps from GPX data...

I use Runkeeper a lot, for tracking all my runs, walks and cycles. (in case you don’t know, it’s a smartphone GPS app that tracks your movement/speed/distance etc… and fully maps the route)

What I’d like to do is to plot ALL of my activities for the past year all on top of each other on one map, to see the full extent and coverage of a year’s worth of exercise – both for interest, and to see if I can then spot new routes around areas I haven’t gone yet.

I googled it a bit and found this page, which is pretty much exactly what I’d like to achieve – a heatmap with all runs showing on my local area.

I’ve exported my tracking data as GPX files from Runkeeper and downloaded the first piece of recommended software TileMill, but that’s as far as I’ve got. I have no clue how, even though it’s explained on the link, to complete the next stage. I haven’t the slightest clue about coding anything, hence my confusion with the program.

So, firstly: is there another, easier way I can overlap all *.gpx files on one map and view them?

If not, has anyone used either of the two programs mentioned, or have experience of other similar programs that could also produce a similar result? Or could anyone explain (in terms a coding-deficient idiot like myself could understand) how to create a shapefile and then a map as per the stages in the link above? I’m clearly missing something fundamental in the whole caper.

Many thanks for any assistance you can offer.