The trail from Whitney Portal is very straightforward walking, agree. Even in September, there were a couple spots of ice on the switchbacks (easily avoided) and we got a surprise half inch of snow at trail crest. But no traction required; the trail is quite well graded.
Bigger concerns for Whitney are:
>It's a long walk as a day trip (22 miles round trip, 6K elevation gain), and there are only a couple camping areas en route.
>High elevation.
>Required permits.
We handled these as follows:
>We made sure to be in good shape so we could do a day trip (ADK Great Range Traverse at 25 miles and 9K is a good training hike).
>We put together a good acclimatization schedule: Sleeping at 8K in Tuolomne; Tenaya (10K); Hoffman (11K); Dana (13K); rest day; then Whitney.
>We got very lucky and got a day trip permit on the date we wanted.
I understand the reason for it, but the red tape is a pain. (Camping permits are more of a pain, although we skipped that.) Also a pain was picking up your permit in the early AM (3AM). Very little signage and nobody home at the permit station, prowling around the outside of the little building in the dark trying to figure out what to do... Also a pain was finding the trail start, once parked at the Portal. Tons of commercial crap there with huge signs about giant pancakes, etc; the trailhead is a little unlit brown-on-brown sign off to the side; took me ten minutes of my hiking time prowling around the lot in the dark to find it.
You can certainly handle the fitness and the acclimatization on your own. A guide service may be particularly helpful in handling the red tape.