Sentence:  ... I had my Health and Limbs ... and might, with Ease, travel a Day or two on foot, and having a good Certificate of being in perfect Health, might either hire a Horse, or <WSD>take Post</WSD> on the Road, as I thought fit. 
Meaning of the word marked with <WSD> tags:  To travel (somewhere) by posthorse or post chaise; to go (somewhere) as quickly as possible.