Wharvey,
You can take the bus that runs LGA/125th street, and take that to the subway.
It's a free transfer. Your cost is 1 fare.
https://bustime.mta.info/m/?q=M60-SBS
Take the bus to Broadway and 116th street, where you can catch the train to Penn Station. It's a 30 minute ride to Lexington, so add time to 116th street and Broadway stop.
The other option is to take the Q33 bus to the subway (E train) to Penn Station.
As noted below, this is no longer an option.
More here >
https://www.tripadvisor.com/ShowTopi..._New_York.html
I'm sure others will chime in.