Get a Research Assistant job in your field and take whatever classes you can, either with the undergrads or at the Adult Studies/Extension school (often you'll get tuition help for being an employee). Then bust your ass.
I did this when I wanted to switch into computational biology from CS without any life science experience. The lab was understanding that I was taking classes on the side (others in the lab did it since RA jobs are often used this way). I took classes with the undergrads, which made it considerably harder, but it was worth the stress and lack of sleep.