I think the 10,000ft "target" is more of an indicator of the amount of work/effort you need to put in rather than something you must achieve at all costs.
Different people need to do different things to get into shape, there's no one size fits all training regime.
Most fell runners don't spend a lot of time actually running/racing - looking at last year's Ennerdale race results (one of the longest actual fell races) , most finished in under 6 hours, i.e. a quarter of the time you are likely to be out on a BGR. Most races are far shorter than the Ennerdale.
There's little point doing 100 miles a week if they are all along the canal bank so some ascent/descent is required. I probably averaged 5000ft and around 25miles/week when training for my BGR.