Snowdonia. The classic route here is the Welsh 3000s that traverses the 15 peaks over 3000 ft. [Distance 20 miles, ascent 10000ft: from first peak to final peak]. I've completed this route approxiametly ten times: going at various speeds that range from 7 hours to 24 hours. Various accounts are given HERE.
The Lake District. A favourite route is the classic Lakeland 3000s route. This links the 4 major summits over 3000 ft. Unfortunately the route includes a lot of road/flat walking: however you can make it more interesting by going over various additonal ridges such at the ridge including Cat Bells and the Helvellyn ridge. [Distance 45 miles, ascent 10000ft for most direct route.] A route description is HERE.
Additional routes in the Lakes/Snowdonia See HERE for more accounts of various long routes attempted in spring/summer 2009: each extended versions of the Welsh 3000s and Lakeland 3000s. Going on to more ambitious levels, there is The Lakeland 42 peaks: Readers of the `Hitchhikers Guide to the Galaxy' will recall that 42 is the answer to life, the universe and everything. However in the Lake District the number 42 has another special significance. It is the number of peaks on the classic Bob Graham Round. Historically this is the vigorous extension of the Lakeland 3000s. The closet (so far) I've come to completing this is reviewed HERE. Traditional measurements give 72 miles, 27000 ft. I'll be back though...
Scotland: Tranter's Round This is a classic trip that includes 19 Munros in the Fort William area (thus including Ben Nevis). I haven't personally documented my adventure, but see HERE for an overview. My attempt was in 2013, and successfully completed it in about 17 hours (solo, unsupported, onsight). A faster time might have been possible, but the final slippery path down Ben Nevis in the dark added on an extra hour. [36 miles, 20000ft].
Scotland: The Knoydart 7 This is one of the remotest parts of Scotland. This includes the (mainland) village of Inverie which is cut off from Scotland's road network, and you must arrive by boat: otherwise it's a 15 mile walk over rough mountains to the nearest road. This area contains 7 distinct Munro peaks, and a natural route links them all together: but it's rough. A personal account is documented in a 2015 issue of Fellrunner, see also the links at the bottom of the following page: HERE . [35-40 miles, 15000-17000ft depending on routes taken, approx 15hours30.].
Cornwall Coast: Done as a day trip from Exeter, an excellent route is the South-west Coast Path route from St Ives to Penzance (via Lands End). [Approx 40 miles, 8000ft.] The route is very variable in terrain ranging from rocky cliffs, old mine workings, grassy ridges and sandy beaches. When I undertook the route over a Bank Holiday Monday, I took the first train out from Exeter to St Ives (arriving 10:30am), and returned on the night train from Penzance (leaving 9pm): thus had a 10-11 hour time limit to get all the way around. For a 300 mile round trip, it was also one of the cheapest train journies involving a cheap-day return ticket: costing 15 pounds (in 2013) with first class seating on the return (!).