Your distance can easily be ascertained from an OS map using the 1km squares, or a piece of string.

Height gain is slightly more tricky to calculate as it will involve the careful reading of contour lines. At this point I issue a warning. As good as McNeish's book is, a large number of the height gain figures are wrong. So how do we do it? Firstly add up the altitude of all the summits and tops you intend to visit. Then sum all the low points in the same way. By 'low points' I mean your starting altitude and all the bealachs (dips) between the tops. Subtract the second number away from the first, add five to ten percent (for all those pesky undulations that can't be seen on the map) and that will give you your total height gain.

Here's an example from McNeish's book. It's a 21km walk that takes in three Munro's and a Munro top. Sgurr na Ciche has an altitude of 1040m, Garbh Chioch Mhor is at 1013m, the intermediate top at 968m, and Sgurr nan Coireachan at 953m. Add those up and you get 3974m.
The starting point, at the carpark at the end of Loch Arkaig, is at 60m above sea-level. The bealach between Sgurr na Ciche and Garbh Chioch Mhor is at 850m. The next dip is at 940m, prior to that top, Garbh Chioch Beag. We then descend to 730m, before the final summit of the day. Add those up and you get 2580m.



The difference between these two values is 1394m. Add 5% (70m), round it up to three sig figs and we'll call it 1470m and that is your total height gain for the day. If you compare this to the description of this walk in McNeish, you'll see that he gives 914m. That's a big difference.
Finally, we apply Naismith's rule. One hour per 5km plus one hour per 600m of ascent. Twenty one kilometres, divided by five equals 4.2 So that's four hours and twelve minutes for the distance. 1470m divided by 600 equals 2.45, so that's about two and a half hours for the ascent. Add those together and you get just over six and a half hours. Smith's rule (that's me) says to add 25%. That is for admiring the view, taking photos, adding and removing layers, catching your breath, having a drink, lunch, navigating and so on... So eight hours.

There are many clever electronic 'apps' and devices that will do this for you. But where's the fun in that? Learning map-reading skills is essential for anyone who intends to spend time in the mountains. It could be life-saving!