In terms of distance, that probably depends on whether you count the Beeston Bump and Roman Camp (on the North Norfolk Coast) as hills. If so, then something over 70km; I've visted them both on a couple of separate walks of similar distance (a long while apart), though was almost hobbling and exhausted by the end of the second route.
Presuming that those don't count, then 44km is my longest day-walk involving significant (Scottish) hills. As well as being the most ascent and, unsurprisingly, the most hours of walking in a single day (about 17.5, though that included hundreds of photos). I only managed 1 or 2 Munros for each of the next 2 days
Having fair-length legs and being used to occasional long (flat) walks, I enjoy doing a fair distance in the hills. It's steep ground that slows me down, especially descending it.