No need to muck around with the GPX (unless you want to): Just download the free Singletracks app (Android or iPhone), login to your account, and you can view the maps and even cache certain maps for offline use before you hit the trail.
However, if you have a GPS app on your phone you already use that is GPX compatible, the process will generally involve downloading the GPX file to your computer and emailing the file to yourself so you can open it on the phone. It’s definitely a hassle, though no more or less so than with many dedicated GPS devices.
Seems like someone wrote up a step-by-step tutorial on this on the forums a while back but I wasn’t able to dig it up…
While viewing tracks in the mobile app is ok, I’ve had problems with my phone showing my curent location on map at the same time. So, You can download gpx from web sight in browser, ether on comp or phone. The app I use to open those is gpx viewer.