The access is about a 60 yard hike with the kayak from the parking lot across the bike trail and as you cross you will see to the northwest a short path to a nice landing and take off point. This is a very nice little workout inside the city. If for some reason you have to bail the flow could be a problem... have a rescue line ready or better yet.. do it with a friend or two.
There is a canal road that is dirt directly adjacent to the canal so you could have someone tag along on a bike with a throw line to help you get out if necessary.
Don't go south of the I-40 bridge since the Ditch empties with a gnarly drop structure into the Rio Grande. If you want to go further north of the Rio Grande Nature Center you will eventually hit the crossing and mini rapid at Montano Bridge. You could get out and portage this obstruction and continue on north of Montano Road to Paseo De Norte and then turn around and go back but most of the time the couple of hours paddle will be fine. Visually check the ditch. Some times of the year the flows will be too shallow for an enjoyable paddle.
The Clear Ditch is conveyance canal that runs most parts of the season. It does on occasion have some low lying branches that can catch you and your hat, but most easily avoided. It also has some steep sides so watch for streamside exits as you go up it.
I traveled today for about 2 hours going north against the flow up to the Rio Grande State Park Nature Center before turning and returning.