Level change works that way. Aside from VNAV, it's mostly used in conjunction with climbs/descents exceeding 1000ft (or else you'd use vertical speed)
Level change will maintain the selected MCP speed while climbing or descending to your selected MCP altitude. Speed ALWAYS have first priority, that's why your aircraft starts to pitch up; it's trying to decrease speed. When that is achieved, your descent will continue. Obviously, if you enter an MCP speed that's is far below your current speed, the descent will take longer to initiate.
My advice is that you enter some altitude, but more importantly: speed constraints in the FMC, so in VNAV mode, the aircraft will decelerate by itself. On the ND, you'll now have a green text labeled "DECEL" on the magenta flight course.
Remember that manual inputs will always have first priority in the FMC. The computer calculations can be abit off sometimes, so it's important to monitor the descent.
Also, remember you can use the airbrakes to lose some speed!
Personally I like to use the LVL CHG over VNAV in descents, gives you a little bit more control. I know many pilots don't like the vnav in the real 737, it's not exactly smooth
Good luck!
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