Some cats tolerate gradual harness training, but many find restraint or unfamiliar outdoor places stressful. A Maine Coon's reputation for being dog-like is not evidence that it wants walks. Do not force the activity or assume that remaining still means the cat is comfortable. If you are considering it, discuss suitability with your vet and introduce equipment slowly indoors, using a cat-specific harness rather than attaching a lead to a collar. Observe the whole body and stop when the cat shows discomfort.
A secure familiar outdoor enclosure may be a better option for some households. Walking should never become a requirement that the cat must meet to receive enrichment. Indoor play, exploration and appropriate furniture can provide alternatives. The cat's response, not an appealing social-media image, should decide whether harness activity continues. Make the first goal relaxed contact with the equipment indoors, not reaching the pavement. Progress should follow the cat's behaviour rather than a fixed outing date.
For product comparisons alongside this advice, see Paw-Villian cardboard scratcher and Mouse Chase play set. The details below explain their sizes and uses.
Further reading: rspca.org.uk; cats.org.uk; pdsa.org.uk; pdsa.org.uk.
Paw-Villian cardboard scratcher
Mouse Chase play set
Koda Pod