It doesn't take long to start putting in distance. When I moved to ATL from the Chicago area a couple of workers decided to take me for a little run. I lasted almost 5 miles then the HR just spiked 180bpm. That was in Aug. In November I ran my first Half marathon and scheduled for another 1/2M in the beginning of March and Tri (sprint) mid march, and a full Marathon at the end. I find it easier to go "running" than the biking which I just started doing. You might think you're going slow but as long as the HR stays at a appropriate rate you're burning calories.
If you were 160 when competetive you're another 50% heavier. Just go slow since an injury will really slow your progress