Start With the Numbers, Not the Photograph

A photograph of a still tarn under a ridge tells you nothing about whether you can get there and back comfortably. What matters is the arithmetic: how far, how much up, and what the ground under your boots is actually like. Most guidebook routes in the Lake District, Eryri, the Pennines and the Scottish Highlands give you distance and total ascent, and those two figures together are a far better predictor of effort than distance alone.

A useful rule of thumb is that 100 metres of ascent costs roughly the same effort as one kilometre on the flat. So a 6 km walk with 500 m of climbing is not a 6 km walk at all — think of it as an 11 km walk. That single adjustment explains why so many people who can comfortably stroll 12 km on the towpath find a 7 km fell route unexpectedly brutal.

Reading the Three Figures Properly

• Distance — measured on the map, so it ignores all the zig-zagging you will do on a steep pitch. Add 10–20 per cent for anything on loose scree or eroded path.
• Ascent — the honest measure of a fell day. Under 300 m is a gentle half-day. 300–600 m suits a reasonably fit walker. 600–900 m is a proper hill day. Above 900 m, you need recent experience and a good weather window.
• Terrain — grass, slab, boulder field, peat hag, or pathless heather. This is where the guidebook's grade often hides the truth.

Compare the totals with a route you already know. If you did a 9 km, 400 m circuit last spring and felt fine, a 12 km, 700 m route in the same area is a sensible step up. Doubling both figures at once is not a step up; it is a different sport.

Match the Route to Your Recent Walking, Not Your Best Ever

Be honest about the last three months rather than the last three years. Fitness fades quickly — a fortnight of desk work and short evenings will cost you more than you expect on the first steep pull. If your recent walking has been valley paths and low fells, start there. If you have been out on consecutive weekends with 500 m or more of ascent, you can look at a longer ridge day.

Time on feet matters as much as effort. A route that takes five hours including stops needs food, water and a realistic turnaround time. Work out your likely pace, add an hour for a swim and lunch, and check it against daylight with a generous margin. In the northern hills in November, that margin is not caution — it is basic planning.

Groups should plan for the slowest member, not the average. A route that suits three of you and exhausts the fourth is a route that suits nobody.

Terrain Changes Everything

A grassy 700 m of ascent on a good path in the Howgills feels nothing like the same height on the boulder fields of Y Gribin or the loose schist of a Highland corrie. Consider what the descent will be like too: wet grass above a stream is more tiring and slower than a pitched path, and it is where most slips happen.

• Good paths and pitched sections — allow a steady 3 km/h uphill, faster down.
• Rough grass, heather and bog — nearer 2 km/h, sometimes less after rain.
• Scree, boulders and pathless ground — 1–1.5 km/h on the way up, and no faster down.

Anything with hands-on-rock sections deserves a separate judgement. If the route description says "a short scramble" or "care needed in descent", add time and decide whether you want that on tired legs at four in the afternoon.

Plan the Tarn, Not Just the Summit

Many of the best fell days in northern Britain end at a tarn rather than a trig point, and that is a sensible objective in itself. Easedale Tarn, Stickle Tarn, Blea Tarn above Langdale, Red Tarn under Helvellyn, Llyn Cau on Cadair Idris and the little pools scattered across the Cheviots all offer a swim without the commitment of a summit.

If a dip is part of the plan, factor in the extra time and the extra kit: something to dry off with, a warm layer for afterwards, and footwear you can get back on cold feet. Water in these tarns sits between about 8°C and 15°C for most of the year, and the shock on entry is real regardless of how fit you are. Wade in slowly, keep it short on a first visit, and remember that a swim costs energy even when it feels wonderful.

Choose a tarn with a safe, shallow entry and a straightforward exit — gravel or a gentle grass shelf rather than steep, slippery rock. And leave yourself a reasonable walk-out afterwards, not a brutal pull straight up a scree slope.

Keep Something in Reserve for the Way Down

The descent is where tiredness shows. Knees, ankles and concentration all fade, and the last hour of a fell day is statistically the most likely time to trip. A route that leaves you pleasantly tired on the summit and comfortable on the track back is a route chosen well.

Practically, that means picking a route roughly 20 per cent easier than the hardest thing you think you could manage. Aim to finish with energy in the tank. If you are unsure between two options, take the shorter one, swim longer, and walk out in daylight with a cup of tea waiting. You will come back, and next time the bigger route will feel like an obvious choice rather than a gamble.