I'll just add my two bits: You are doing a great job! Your baby is just going through a growth spurt, and after a day or so (but possibly up to about a week) she will spread out her feedings again (until the next growth spurt). At any time you are concerned about how frequently or how long she eats, just count her wet and poopy diapers and you will know whether she is getting enough. (You already seem to know all this, but being told what you already know is really helpful sometimes!) Also, the sleepiness is totally normal too, and especially during a growth spurt babies seem to sleep more than usual. Sometimes, if you watch closely, you can actually see them grow! :)
Another help: if you haven't already, learn to feed her while lying down. The rest is good for you, and soon you will be able to sleep (at least partially) while she feeds, and you can totally relax. (For me, my baby's bed is mine, so our sleep cycles also coincide. In other words, I go into a light sleep or wake up about the same time the baby does, so when the baby is hungry, I am also ready and not disturbed and we can both happily go back to sleep with the baby feeding as long as it likes. -- see Dr. Sears' book "Nighttime Parenting" he explains what I have experienced with all my babies very well.)
But then, you might ask, about the toddler? While you are feeding the baby, you can read with the toddler, allow your toddler to fetch diapers or other desirables for the baby and save you energy, cuddle with the toddler in a chair or even have him/her lie down with you and the baby (be careful, of course, the toddler doesn't accidentally hurt the baby when getting in position for cuddle-time).
Remember that this time with your little ones is very short -- I was told once, "The days are long, but the years are short."
My oldest is nearly 10 now. I have 4, with one on the way, and I was told that when my first was a tiny baby, and I was feeling much as you describe. I have found the saying to be very true. I have been breastfeeding since the birth of my first nearly without a break (we wean at around 3 years old, so my toddlers nurse through the next's pregnancy and then usually wean a few months after the next's birth). (Of course, they start solids at around 6 months - but they just continue breastfeeding too!) This time of feeding our babies is very precious, and we learn things about our children and their personalities in such a nice way. :)
Also, the leaking milk is also normal at first, and I think it is around 3 months that it doesn't leak so much anymore, even though you are producing plenty of milk, and the breasts get softer, too -- no longer that firm, almost engorged feeling, but still plenty of milk. It is just an adjustment the body makes as it adjusts to your new normal.
Also, being tired around this time is also normal. You have a newborn -- there is care around the clock. You have a toddler -- they need nearly as much (if not more at times) attention than the newborn. You are still recovering from the birth. That was quite an event for you, and your body is still adjusting and readjusting, decreasing blood volume, etc. and breastfeeding helps with all that, too. Not to mention all the emotions...
So, remember to rest as much as possible and focus on the babies (and your husband - remember to thank him for all his great support and care - he will need some nurturing too)and getting enough to eat and water to drink. Be sure to get enough protein, too. Remember that you are still supporting the growth of your baby 100%, even as much as if it was still inside you, only now it is a lot bigger, and getting more so!
You are doing a great job, and it does all work out and "pay off". The effort is worth it!
Hope this helps some - enjoy your babies!...