It seems that Fanny's poor health goes back to her early years. Her Mother describes her as 'delicate' (ie prone to illness) and 'puny' (inferior in size and strength, weak). The reasons for this could be both genetic and environmental.

Poor nutrition could play a part. Mrs Price speaks of her 'fine boys', and I wonder if she fed them better than her daughters. This happens today in some cultures where females are less valued than males, and isn't irrational if family income depends totally on the health, strength and earning capacity of the men and boys.

Mrs P also hopes that Fanny will be better for a change of air. Open coal fires produce a lot of smoke and soot (less of a problem with large airy rooms and an army of servants to keep them clean), which would make her a sitting duck for respiratory problems. The need for a change of air could also imply poor sanitation, which would place an added burden on Fanny's immune system. If she has also had limited exercise her cardiovascular fitness will be compromised.

No wonder that when she arrives at Mansfield aged 10 she is small for her age and has no 'glow of complexion. She has had a poor start in life, and impaired early-years development can have lifelong effects. Her appearance improves after a while, presumably because of a healthier environment and better nutrition.

At age 12 Fanny starts riding and eventually learns to like it, but even six years later she never chooses to go far. I don't ride, but understand that this would be a way of exercising most of the body's muscle groups while taking things at her own pace. I suspect that part of her problem with walking is that she is usually with someone bigger, stronger and fitter than she is. Imagine Aunt Norris as a walking companion, endlessly scolding and admonishing her to "Keep up, Fanny!!".

I think a contributing cause of Fanny's physical weakness could be psychological, specifically her sense of worthlessness and her lack of self-confidence, which frequently includes lack of body-confidence. This sort of physical constraint is often associated with deep-seated muscular tension, which in turn affects muscular development and cardiovascular fitness. Not an easy loop to get out of.