I am often puzzled by the limits people place on riding a folder, puzzled whether those limits are inherent to a bike or whether they are fixable.

If any bike is of limited distance value, then the question needs answering, what is the limiting factor? I am guessing it is fatigue. Fatigue brought on by incorrect fitting of the bike, or unsuitable gearing. As long as those 2 are good, and assuming the bike isn't super heavy or very inefficient, then a rider should not know the difference.

If a bike is limiting due to size/fitment, in many cases that can be fixed. Inadequate gearing is more costly to fix, or almost impossible in particular models. Weight or efficiency can be addressed within limits. A bike weighed down by heavy tyres, mudguards, racks and the like can be quickly put on a diet. Inefficiency due to bad tyres is also readily fixed. A flexy frame will also eat calories and can't be fixed.

Fitment-wise, the 2 models are identical? The D7 will have a softer heavier saddle unsuitable for longer rides; but you may not like the P8's either. Assume you have to replace whatever one you get for longer rides at least. The P8 comes with Big Apples IIRC, that's a pretty good tyre but rather heavy. The D7 come with Kenda Kwest, IMHO an unsung hero but equally heavy. Both cases will do well with Marathon Plus.