The capsules slowed down by turning speed into heat, just link the space shuttle. Parachutes did not deploy until the capsule had slowed (yes, slowed) to terminal velocity. I am not sure how fast the capsules were going when the first drogue parachutes deployed, but one can rest assured it was much slower than 12,500 MPH.
And it would be very, very difficult to separate a capsule from an airplane flying at 12,500 MPH.
There are many other places to put our money and effort right now, rather than reengineering a 30 year old design. Apollo was an idea on the back of a napkin in 1961, it flew in 1968, and landed on the moon in 1969.
I believe we could build a small, reusable capsule or lifting body that could carry 4-6 people atop an expendable Delta IV rocket within 5 years. A small vehicle could use titanium or other materials instead of the tiles for heat shielding.
We then would only have to use the shuttle for lofting big loads to the ISS. We could even modify the shuttle to fly unmanned for most flights. To gain better economics from the shuttle, we could build an unmanned, disposable cargo carrier to use the existing shuttle solid boosters, external tank, and assembly and launch facilities. This would give us a lifting capability near that of the Saturn V, which we would need for future Moon or Mars missions.