As an ATPL theory instructor I can give you the other side of the coin. With your qualifications the standard of material won't be a problem. It's volume in time that's the killer & distance learning (D/L) can be a lonely place.
I have had students complete the ATPL theory in 6-8 months without prior experience but this is unusual. Others are still trying and are on their second sittings after a couple of years. On average if you are putting in around 15 hours per week you are looking at doing the theory in about a year. Please remember that once you have taken your first exam you have 18 months to complete all the theory, maximum 6 sittings in total and maximum 4 for a subject.
Though ability does play a part it is usually lack of motivation or distractions (eg. running a business) that causes students the biggest problems with D/L. I have conversations every month with students who are about to expire in terms of time & or sittings for whatever reasons (you don't want to be here). On a positive note most students do get through first time within the time limits but don't underestimate the work required - it is not an easy option.
Best of luck.