This is impossible to answer unless you are willing to do an enormous amount of research. There is no single, or even just a few, databases of trails in the US.
The national trail lists generally only contain the larger multi-state trails (like the AT, PCT, etc) and some of the larger regional trails, but these are the tip of the iceberg. I'm guessing, but the total length of trails is likely 100s if not over 1000 times more.
For example, here in Massachusetts, we have the AT running thru the western end of the state, and some locally well-known regional trails, like the Bay Circuit, Midstate, and M&M trails. Look around on some national lists and see how many of the regional ones are listed at all.
Then consider that the bulk of trails in MA are actually much shorter trails in individual towns. The Midstate Trail runs north/south thru MA, connecting NH and RI, and is 95 miles long. Here in Groton alone, we have over 115 miles of trails, and that's just one town. Groton may have a larger trail system than most, but even if the average is only 25 miles per town, that still dwarfs the lengths of the national and regional trails in MA. However, you're very unlikely to find more than a tiny fraction of these local trails listed anywhere nationally.
This state of affairs is slowly changing, but even that is happening piecemeal at the local, regional, and state levels. A comprehensive national database is decades away at best. Even an MA state-wide database is years away. There have been individual efforts, but none comprehensive yet, although there is at least some talk about that. DCR (the MA Dept of Conservation and Recreation, owners of the state parks and forests) has now just within the last couple of years compiled a list of trails on their properties. Some of the regional planning agencies have tried to compile lists of trails within their regions. For example, the Montechusett region (roughly north central MA) has a region-wide trails map, but that doesn't contain every last trail in all the towns. It's also too low resolution to make that practical anyway.
Groton does have a comprehensive map of all trails in town, but no really definitive measure of all their lengths. Our claim of 115 miles is an estimate based on reasonable assumptions, but not on comprehensive accurate measurements. I'm on the town Trails Committee and am actually working on that. I'm going around measuring all our trails with a measuring wheel, and taking GPS tracks at the same time. I'll eventually get to all of them, but that will probably be another 2-3 years out. And, even if I say so myself, we're well ahead of most towns in the state. Most don't have Trails Committees, so local land trusts and other private organizations and "friends of ..." groups maintain their specific trails, with not even a town-wide website, master list, or any one group responsible for such things.
The reason I'm telling you all this detail is to put the problem in perspective. To get a complete list of trails in MA, you'd have to gather information from several federal and state agencies, dozens of local land trusts, and various other official, private, and volunteer groups in each of the 351 towns and cities. After you're done with that, you only have the remaining 99.72% of the US land area left to do.