Not possible with Garmin Connect at this time. I wouldn't expect to see it either; something as simple as tracking shoe mileage has been requested repeatedly for years.
I use RunningAhead.com to create training plans. The site has a great tool to make your own custom plan. You can set it up to email you reminders of your workouts, and/or you can download the workout as an ICS calendar. No way to put it into the watch as far as I know.
You can create individual workouts, and add them to the calendar each day that you want to do them. It is tedious, but it does work and the workouts can be uploaded to your device (assuming that you're able to upload anything to your device). Unfortunately you can't create a full training plan (say a 8 week 5K training plan) schedule all of the workouts as a block.
You can create individual workouts [...] It is tedious, but it does work
True. I didn't mention this method since it is so tedious. Especially if you're building something like a 12 week stepped plan where each week's workouts get longer or more intense or have more interval repeats and you end up with 25-30 different workouts. Probably better to just edit and schedule workouts on a weekly basis to make it more manageable. But then you lose any ability to tweak and re-use the training plan for another race later on. It'd be nice if Garmin provided some real functionality here.
One additional tidbit to be aware of on the 610 (not sure if it affects other Garmins): you lose auto-pause capability when running a custom workout and will need to pause the workout manually. (importance of this depends on the runner and his/her typical routes)
