Age is only a problem when you use it as an excuse!
Pasture ground cut up by hooves then dried out is probably the worst ground to have to cover. As others have said, choose the bike for the most common type of terrain: The YD300 has about 500m of technical ground, there’s no point in using a DH oriented FS as it’s unsuitable for the 120Km of road and 120Km of vehicle track elswhere on the route, even walking the hardest descent only costs you a couple of minutes in an event lasting 30hrs.
There’s a huge difference in riding a bike off-road for a few hours vs riding that same bike for 100 miles or most of a day. A bike might be fine comfy for short rides but any (and I mean any) shortcomings in your setup will be exposed on the longer rides. My HT, as initially setup, was fine up to about 100Km then was painful. It took a few long rides over the course of a year or so to get everything right.
To the OP: if you are on a FS now then use that, it will take you a while to get used to riding a HT over long distances. If you are on a HT then consider that first then if you think it’s not for you get a short travel FS, something like the Salsa Spearfish, aimed at this sort of thing. An alternative might be a ridid or HT with Plus sized tyres to take advantage of that softness of ride.