What kind of charities do you give to? I ask because generally local charities are the most beneficial to your local community and often times those charities can benefit from getting the time of an unpaid volunteer as much or more than they can benefit from you sending them money. If you volunteer 500 hours, that might be the equivalent of them not having to hire or contract labor worth $10,000, for example. If a national charity, then yeah, often times 30% pays payroll, 20% goes to marketing/overhead, and 50% goes to the recipient. That's still not bad, and in many cases it's more efficient than government programs. Still, it's only a portion that makes it to the recipients.
If you're donating to an overseas charity, assuming it's well managed, then the money may go extremely far. Look at World Vision, the Bill & Melinda Gates foundation, Blood:Water, or many others that operate outside the US. A lot of them have only a very small management team in the US and then the rest of "payroll" is basically people paid at the peanuts rate that labor costs in developing nations. Need a paid, full-time worker? That could be $36k/year + taxes & benefits cost in the US or $1100 in Uganda. I didn't make that up either, that's the
actual average wage in Uganda.
I know a few people that look at charity that way; for their local community they volunteer their time and expertise and a limited amount of money. For the rest of the money they send it to respected overseas charities where they cannot physically volunteer but the money could have 30x the impact it does in an expensive country like the USA or Canada. I'm not saying it's right for you, but it's something to consider. If you're retired young, you'll likely still have a lot of energy to go with all your new found time, and signing up as unpaid labor at a charity or two could be a great way to share your abilities with others.