Last spring my 4 year old (who can ride a bike, no training wheels, without a problem) went in the bike seat behind me - the bikes they have available for little kids were a lot heavier than what she is used to, and she was nervous about the terrain (it's relatively smooth and flat, but not for a kid who usual rides a bike on chicago pavement!). She certainly fit in the kid seat - she was 41 inches and 35 lbs at the time. I had one awkward moment when the bike almost fell over when I got out (bc she wasn't sitting still and the balance was hard) but nothing actually happened. I hadn't biked with a kid in a seat since they were little, so it was an adjustment.
I think you should be prepared to do either -- if the bike works on her own, no problem. If it doesn't, easy enough to switch to a bike seat
here's a pretty good overview -
https://www.themouseforless.com/blog_world/2016/07/disney-cruise-line-bike-rentals-castaway-cay/
as for distance, I think if you do the entire circuit it's about 2 miles.