I have an 8 month pup who shows some promise, but is currently moving more narrowly behind than we would like.

I have been advised to do more roadwork with her (she currently gets lots of free running, at her own pace in our fields). Some people have advised us to let her pull on the lead, to make the work more resistance based.

As a total novice to showing, I am intrigued how roadwork works on physiology to improve movement and would be very grateful for any advice or explanation on how it all works.

Many thanks.

What breed is your puppy?

You have to be very careful about not overexercising an 8 month old pup, not sure what breed yours is but especially careful if she is a large/giant breed?

Generally roadwork will improve/tighten up feet, any exercise at a constant pace will improve muscle tone which improves movement.

She is a whippet. :-)

As far as exercise goes, I don't take her for long walks (trying to follow the 5 mins per month idea). She does have a lot of access to our very large garden though and when a whippet want to run......

I think that the physiology of this is that road walking helps develops the gluteal muscles which act to keep the legs wider.  This only will exercise her gluteals if she is maintaining a one, two, three, four pattern of walking, not ambling or pacing etc.  Its complicated!
I'd have a try and 'walk' her but also suggest you teach her where her back legs are by taking her over very small poles, or walking every day through poles or logs thrown down like pick up sticks - not too much at 8 months and of course walking her on both sides.  Have a look up of caveletti pole work for dogs or similar.
it is amazing how many dogs have no idea their front and back are connected.
When puppies learn to use all four feet independently, and know where they are it is great to see how confident they become physically.   Well mine do!