I took up tennis when I was 27---closest thing I could find to baseball without having to round up 17 other guys. Been playing for over 35 years and, at one time, played on a 'semi-pro circut' here in Southern California. It really is a sport that you can play for most of your life.
One thing that has always been clear to me is that you can't play tennis to get in shape---you have to get in shape to play tennis.
Check this site for free video instructionals that will arrive via e-mail 3-4 days a week: www.webtennis.net
It's short lessons and/or talking points about strokes, strategy, conditioning, etc. Usually run just 3-5 minutes and you should be able to save them and build a library to refer to. It's authored by a guy named Brent Able who, though he may not be a household name, is highly regarded, both in California and by players of my age. As recently as last year he won a national singles championship in his age group, represented the US against Australia in Men's doubles and, with his wife, finished second in a couple of national MxD championships. He does sell product and I've bought some (which I found helpful) but he's not pushy. IMO, it's worth a look. BTW, he's a nice guy too---we know some of the same people on the professional tennis side of the table.