If you are going to do this (which I would not recommend, and I have done more than my share of crazy-long transit reverse commutes), you'll want to get Transitlink cards, available from commuterdirect.com--those are unlimited monthly passes for MARC that also provide unlimited trips on Metro and most local bus systems (though a lot of the bus drivers have never seen one before, so expect to have to argue that point a lot). One that covers the Union Station to Penn or Camden Station trip will cost you $277.

Be aware that Baltimore's subway (which is not the same thing as its light rail system) does not link directly to MARC; you will need to take a bus transfer or walk about half a mile to the nearest subway stop. Your best bet is probably to ride Union to Camden and then walk directly to your office from Camden; it should be about a mile. Otherwise you're looking at Bus->Metro->Commuter Rail->Bus->Subway, or something ridiculous like that, and with that many points of failure something is pretty much bound to go wrong at least once a week.