Though not through Athabaska, I did take 2 Distance Education courses over the summer while on OJT. My experience is as follows:
First, for the sake of understanding, both courses were math courses, and thus were paper submissions via mail, and late submissions were not allowed (date submitted is determined by postmarking).
Honestly, a full-time work schedule does limit how many courses one can take concurrently. I took 2 and my grades were fine but could have been better, and I certainly could not imagine taking more than 2 while working full-time. Schedule wise, I did have some free time, though I did miss the odd TGIT/TGIF due to an assignment deadline. Luckily, I can study in front of the TV, so it wasn't all bad. Personally, I've found full-time course loads in sit-down classes to be more manageable than full-time work and part-time studies.
Another note is that if you're scheduling to write your exam at a W/BSPO office, make sure you schedule it well in advance. I scheduled my exam write date about 3 weeks prior, and I got bit by the WPSO office invigilator taking leave starting the day I wanted to write, and thus resulted in me writing my exam a day early... with 1 days notice since they couldn't find a replacement.