PS. It as been said above that it would be "easy" to falsify a .gpx file. That's true, but I think it would be quite hard to falsify one convincingly.
Easy is relative. I guarantee that I have more chance of convincingly falsifying the GPX tracks than I would of riding 29000 miles in a year.
Think about it: You cook up some software to generate pseudorandom tracks (complete with HR data that mimics your own under similar conditions) on pre-determined routes. Then spend a while hand-crafting some routes, complete with weather fudge-factor, random stoppages, deviations for hedge inspections and so on. Probably day-by day to allow for real-world conditions.
It's work, but a couple of orders of magnitude less work than going out and riding it all would be.
Of course for authenticity you want to be seen to do some actual riding. So your algorithm needs to be reasonably consistent with the real data generated by that (or your riding needs to be consistent with the fake data generated by your software). Which means you need plenty of testing, and a database of equivalent style rides to work with. It's not something you'd hack up the week before the attempt and expect to work.
Live tracking is harder. To fake it convincingly you need to use a commercial system like SPOT that won't arouse suspicion, which probably means reverse-engineering their hardware to inject your fake GPS signal. Doable, I'm sure, but that means more dev time.
And you need to not be caught out by being provably somewhere else when you're supposed to be slogging into the fenland hair-drier or depleting the Marsh Gibbon Premier Inn of
CAKE reserves. That's not trivial, and combined with the above probably represents a similar level of commitment to actually going out and riding the thing.
The question is: Why bother? As a hacking exercise, to show that it can be done, sure. But it's effort that would better be put into, say, developing a more secure tracking system. To convince the world that you've beaten a record, and then keep schtum? Where's the glory in that?