what do you think would happen should I get to close to stepping on them. How likely are they to bite an ankle & what would the effects be? I am starting to get paranoid!

the info page.
Chances are the web will stand out, the widows retreat is in a hole in the ground or under an object, and it will retreat at the first sign of a disturbance, should it be in the web. Even when in the retreat, widows may cower into a corner if the web is being tangled with by something other than an insect or spider.

Under normal circumstances, the spider will not be in the web during the daytime unless it is capturing prey. Abnormal circumstances might be an ant invasion or spider wasp that caused the spider to evacuate the retreat and run out into the web.
Otherwise, most activity takes place during the night. You might find it interesting to use a flashlight and observe the spider in it's web.

Somehow the spider would have to get dislodged from the web/retreat and on your ankle for a bite to occur. Highly unlikely, but I cannot say impossible.
I would not be paranoid, just aware. It sounds like they are not too abundant in your immediate area if you have only seen two.

Hi Jeff,

I've never seen them near my house - but this is the 2nd one i've seen in just 2 months while out shooting photos of wildflowers. I used to watch out for snakes - but now I'll be keeping an eye out for red hourglasses!