The single r can indeed sound a lot like the English "d." Except at the beginning of words (where it is trilled), a single r is formed (more or less) by hitting the tongue against the front of the palate. It is sometimes said that the Spanish "r" sounds like the "tt" in "little," so you're hearing correctly. The exact pronunciation varies somewhat with the speaker, the region the person is from, and the placement of the letter in the word.
When you say the word 'throw' you have to put your tongue in almost the same position as you do when you make the Spanish r sound. Try that for positioning your tongue, then just blow real hard and your tongue will vibrate like it's supposed to do for those rolling rrs. Once you have the tongue vibrating, make a growling sound like 'rrrrrrr.'