It shouldn't be a problem, but you lose accuracy doing it that way as tracklogs saved to the card have a maximum of about 500 points. The internal tracklog can have 10000 points and can last for a couple of days, depending on use and the amount of time that you are active each day.
To make the best of the internal tracklog, go to the map page, then Menu>"Setup Map" and use the cursor keys to select "Map Setup - Tracks". On there, I suggest that you use the following settings:
Saved Tracks - Auto
Track Log - Auto
Track Ponts - 10000
Depending on what you are doing, you may want to experiment with setting the values for Saved Tracks and Track Log to specific distances.
Of course, there's nothing stopping you saving the routes to the card, and then pulling the live log out of the unit when you get back to a PC.