See the table below...
It is true that extra climbing adds severity and that extra distance also (obviously) adds severity. For fellrunning, it's the combination of the two that adds up to a 'killer' race.
Orienteers usually reckon that 100m on the flat is equal to 10m of extra climb (or vice-versa) OR each extra mile = 500ft of climbing. Using this as a basis I looked at a range of races and calculated a severity rating by the formula ft climb/500+distance. The result was this:
Event
Climbing
Distance
Severity
ManxMM
8000
32
48.0
Wasdale
9000
21
39.0
Ennerdale
7500
23
38.0
3 Peaks
4500
23.5
32.5
Duddon
6000
20
32.0
Borrowdale
6500
17
30.0
Arrochar Alps
7800
14
29.6
Langdale
5000
14
24.0
Tour of Pendle
4200
17
25.4
3 Shires
4500
13
22.0
Long Mynd
5000
11
21.0
Grisedale H
5000
10
20.0
Sailbeck
5000
9.5
19.5
Ben Nevis
4400
10
18.8
Snowdon
3300
10
16.6
Ingleborough
2000
7
11.0
Scafell Pike
3000
5
11.0
Stanbury
1250
7
9.5
Dale Head
2200
4.5
8.9
Paddys Pole
1250
5
7.5
It does tend to rank the races pretty accurately (if my personal experiences are anything to go by!). Possible anomalies are that the MMM is not REALLY much tougher than Wasdale (despite more than 50% extra distance) and Scafell Pike should surely rate a higher severity than Ingleborough.
I should also add that the distances are the "official", quoted figures and we know that these are notoriously inaccurate. Duddon would drop down a fair way if it's 'true' distance of about 17 miles were substituted.
Anyway - the aim was to put the 3 Peaks where it belongs and this would seem to do that fairly well, though why I bothered I'm not sure...
Anyone got a better formula that can be demonstrated to rank races more accurately than this?