Walking in Sequoia?

My wife and I are keen walkers in the UK. In late July we have an 8 night
tour to plan, starting and ending at my son's home in Encinitas. Initially
we were thinking of including some hikes in Joshua Tree NP, which I
enjoyed on my solo trip in February 2011. (And we plan to visit Palm
Springs Aerial Tramway in any event.) But a look at the likely Jul/Aug
temperatures in Joshua has quickly changed our minds about that!

We're therefore considering the alternative of getting trains to somewhere
like Monterey and after a day there hiring a car and then staying close to
Sequoia NP (e.g. Three Rivers) for 2 or 3 nights.

What is the likely daytime temperature in the park please? Presumably
hikes of say 5-10 miles are possible on largely shady trails? Over here in
the UK we'd regard 25 C (77 F) as tolerable, but 30 C (86 F) too hot.

And what about this awful Poison Oak I read about? Is it prevalent in that
region?

Any advice would be much appreciated please. Including alternative
suggestions.

--
Terry, East Grinstead, UK