definitely walk. and DEFINITELY take 8th avenue over 7th.
(walk QUICKLY, if it's really more like 10:25p.)
i ride ramtrak ALL the time (i even have status -- "preferred selct" ... you all may admire me!). track access is much closer to 8th avenue than to 7th ... everyone goes in @ 7th, but the trains are @ 8th. plus the crowds are slightly more manageable at street level walking down 8th ave, and you will definitely lose time trying to negotiate once you are *in* penn station if you enter @ 7th. the 7th avenue escalator alone is a disaster ... then you have to get across the entire concourse to 8th avenue where most track access is located.
(there are a few track access points underground b/t 7th and 8th avenues, but you have to know: (a) where they are; and, (b) what track your train is leaving on ... unless you take the same train EVERY DAY, you cant know the latter.)
i also think you are a little tight taking the subway. when you actually factor in the time to get up and down, PLUS the variable of a just-missed train, i definitely say hoof it.
and at intermission warn the people b/t you and the aisle that you will be knocking them over to get out the door. seriously, you have no extra time once the curtain drops! forget the nicities ... skip the applause -- you have no choice.
dont be fooled ... you can do this, but your timing is VERY, VERY tight. if you can swing it, scope out your route from the station entrance at the SE corner of 8th and 33rd right to the tracks BEFORE the show. find the monitors so you can check your track number as soon as you arrive. then head right to the track. it will help to know what youre doing in advance.
not sure whether youre headed north or south, but your train is probably not originating in NY (more likely DC or boston, depending.) your train will be well en route by you intermission, and in theory you could check status at half-time to see if by chance it's running late (the Regionals frequently do). however, the information in my experience is not altogther reliable, and trains do make up time (tho rarely do they make up time coming into NYC from jersey or CT).
prolly more than you wanted, but i have been in your shoes many, many times.
good luck and enjoy!
"You, sir, are a moron." (PlayItAgain)