Direct debits are safer and more convenient than standing orders because
(a) your bank guarantees to immediately refund any money which shouldn't have been taken from your account ; and
(b) the amount paid can be varied without having to complete a fresh load of paperwork.
http://www.bacs.co.uk/BACS/Consumers/Direct+De bit/Your+rights/
Agreeing to pay a regular amount (under a deed of covenant) enables the charity to reclaim the income tax that you paid on the money (assuming, of course, that you're a tax payer). So the charity gets more than you give. (The deed of covenant accounts for some of the paperwork).
It seems a little unusual not to have agreed on an initial regular payment but you've no need to worry because your bank will repay any losses. (It's likely that you'll get a phone call seeking to persuade you to increase the amount you originally said you've give).
My only concern is that you might well have signed up through one of the agency staff who recruit donors on behalf of many charities. These agencies make their money by taking a large cut out of what you pay to the charity for the first few months or more. (In some cases half, or more, of what you pay in the first year goes to the agency, not to the charity). If you want to give to a charity (whether regularly or by a single donation) it's always best to contact the charity directly, so that they get 100% of your donations.
Chris