I was in a similar situation. I got a secured credit card.
When I first got the card, I didn't know about debt utilization, and was proud that I paid the statement balance in full each month. So my credit score stayed in the mid 500 for the first year of the card. when I started keeping my card below 10% my credit score increased to above 600.I paid off the last of my debts in December 2014 (it has yet to update on the bureaus though) so I've been working with this visa and my utilization rate to bump up my scores. Paying of old debt didn't increase my score (it hasn't updated as paid as yet). But lowering my visa balance did. I went from 625(E) and 562 (TU) on December 31, 2014 to 630(E) and 657(TU) on March 15. My Equifax actually dropped from 630 in January to 625 February because of high credit balance, so it's just increasing again.
When your credit score is below 600 having a high credit balance doesn't hurt it.