Certainly, traveling with a 72-year old who still hits the gym or jogs daily is a far different experience than traveling with one who is wheelchair-bound, possibly using incontinence supplies, perhaps even oxygen. In either case, time invested getting yourself into better shape is well spent. You'll have an easier time, and more fun, keeping up with the former; and feel far less dependent on strangers to help with the latter.
There are many factors that come into play: when (season, weather); why (shared 'vacation', medical treatment, family emergency)
"The devil is in the details." Learn as much as you can about: any medications they are taking (and do any need refrigeration?), any adaptive equipment they use, any assistance they need with ADL's (activities of daily living: dressing, bathing, etc.) as you can in advance. How will you get from the curb to the airport gate? From the plane to the hotel? From the lobby to the room? In and out of the hotel tub? Will a change of routine / foods / sleep schedule cause a problem? Do they skip taking 'the water pill' so as not to have to stop for the bathroom? (Hint: you will likely not be informed of this in advance.) If so, do you have a plan for what to do about the swollen feet that will no longer fit into their shoes? At any given point along the way - where is the nearest emergency medical provider? Know your own limits - do not, under any circumstances, take on more than you can handle. Hire help for any tasks "above your pay grade." Abandon any hope you may have had that this will also be a vacation for YOU. Likely not. Although sometimes a cruise ship - an 'assisted living that floats" - might fit the bill for nearly all concerned. Consider a backpack for your carry-on. It's easier to manage while pushing a wheelchair than a rolling suitcase. Schedule a few days for yourself when you get home. You'll be happy you did.