If you have a dog yourself, there may be scope for you to ask her to "help" you - for instance, practising training together, or asking her to come along to a Good Citizen class with you for company and moral support. If she has a large garden, could you suggest she gets a few pieces of agility kit for instance to play with/train with in the garden? PAH now sell some fun pieces. Again, present this as her doing you the favour with your dog coming along for some fun.
Is there a teenager in your family/neighbourhood that you can pressgang into helping you? Suggest that said teen is looking for a voluntary project e.g. for looking good on Uni application, because they want to work with animals, Scout award, church project, anything really, could said teen walk the dogs as part of this? Even if this is once a week for only half an hour, it would be something (personally, I would slip the teen some cash as well) 
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Are there any reasons that maybe she hasn't shared with you why she can't walk the dogs? Family illness, caring responsibilities, recent operation or illness ? If there is a genuine reason, a charity like the Cinnamon Trust would be able to help out by providing volunteer dog walkers - she may not know about this service.
Agree you have to tread carefully - you will have to work with this lady so need to be tactful.
Maybe she hasn't realise that professional dog walking services are now so widespread and affordable - this was virtually unheard of thirty years ago and now they are everywhere. If you know of any good local ones, pick up a leaflet, or mention them in conversation casually to see if she had considered it.