Here, my beloved thus heavily worn and comfy motorbike boots serve as a good example. In fact, I've never rubbed them with leather fat, so they finally got cracks and crevices all over the toe-cap. I drive my motorbike all year long, so they have seen a lot of rain and mud. I wanted them to be waterproof again, but some cracks were just too deep for usual methods!
By trying to find out how to repair leather, I've read about a substance called "liquid leather". In Germany you pay 16,50 Euro for 7 grams (Amazon) of that substance... but, c'mon, it's for some heavy duty shoes and not for a Chesterfield sofa!
Recently, I've been installing a big solar cell onto the roof of my car, thus I had some Sikaflex 222UV left over. Instead of opening a fresh flask of fungicide-free aquarium silicone, I've taken Sikaflex to repair my boots. And this is how you can do it for yours:
1. Clean your boots with water and some mild dishwashing detergent. Let them dry for a couple of hours or even a day. It might be a good idea to clean/rub the area around the deepest cracks with some alcohol or sandpaper them for the glue to stick even stronger. But I haven't done that. However, if the crack is a real gap, you'll have to sew it first and make sure, that the rip does not widen too much while you walk.
2. After your shoes are dry, apply some Sikaflex 222UV. It's an uv-resistant, bonding and permanently elastic substance similar to sanitary silicone, just better - as it does not contain any fungicides and does not release acetic acid while curing. I took a skewer to apply the pasty substance, but anything else like an old paintbrush, palette-knife and such will be just fine. The more careful you work, the better your result. I just wanted my shoes to be waterproof again, so my photos won't show you a piece of art ;-)!
3. Remember to push the Sikaflex deep into your cracks! Thereafter, let it cure for about one or two days - depending on how much of it you applied. I recommend two full days for that. The substance shouldn't feel sticky anymore, just rubbery!
4. Apply an overall coating of some leather impregnating agent (any brand you find at your local shoe store will most likely do the job) to the whole shoe and let them dry for a couple of hours.
5. Water test your shoes! Mine were perfectly tight.
6. Keep walking ;-)
By the way: Sikaflex 222 UV is also perfect for glueing your shoe soles and such. Unfortunately, it's only available in the following colors: black and white.