The evil of the actual disparity in their ages (and Mr. Woodhouse had not married early) was much increased by his constitution and habits; for having been a valetudinarian all his life, without activity of mind or body, he was a much older man in ways than in years; and though everywhere beloved for the friendliness of his heart and his amiable temper, his talents could not have recommended him at any time.

Valetudinarian: weakly; sickly; infirm of health (Johnson's Dictionary, 1805).

I don't suppose that being sickly would prevent a person from being married.

He doesn't like London, which he consideres unhealthy; he even laments that Isabella lives there:

"Ah! my poor dear child, the truth is, that in London it is always a sickly season. Nobody is healthy in London -- nobody can be. It is a dreadful thing to have you forced to live there! so far off! and the air so bad!"

Even when Isabella assures him that they live in a part of London "very superior to most others" and "very airy", he is not convinced:

"Ah! my dear, it is not like Hartfield. You make the best of it -- but after you have been a week at Hartfield, you are all of you different creatures; you do not look like the same. Now I cannot say, that I think you are any of you looking well at present."