Angela-
Note to other readers: May contain more information that you would like to read :)
I had my Mirena inserted in May 2009 during my period. Prior to the insertion I had not had a period for over 8-10 months. In fact the period I had during insertion was forced by a cycle of progesterone pills (which was not fun in itself). And just to point out, I haven't had kids or have ever been pregnant. Prior to insertion I was advised to take 600-800mg of Advil about an hour or so before my appt and was also given a dose of misoprostol to take the night before. I believe the misoprostol was to soften the cervix prior to insertion. Prior to deciding on the mirena (or an IUD in general), I had done a ton of research and was scared to death of insertion has I had read many horror stories about pain, cramping, etc. But for me, insertion was a breeze. First they get you all get up like a normal pelvic exam and then they sound the uterus to determine the size/location. My dr did not use any numbing agent for the insertion but I have read that some will. For me, the sounding was not that painful - a quick cramp with a pain level (more discomfort than pain) of about 3-4 for about 5-10 sec. The Dr actually mentioned that they might not be able to insert the IUD for some reason but was willing to try if I wanted them to which I did. And it went fine. The actual IUD insertion was a bit more discomfort - maybe a 6 (for about 5-10 sec) but really nothing more that I had ever experienced with menstrual cramps or even stomach cramps. I just sort of went to that place I go in painful workouts and remembered to breath. In no time, it was over all and was well. After reading all of the horrible stories, I was shocked at how easy and almost pain free it was. I will admit, I did get a little dizzy and had to lay there for a few minutes.
I had plans to take the rest of the day off because I had read about cramping and other things. So I went home and sat on the couch. After about 2 hours of that I was bored. I felt fine and felt like a loser to sitting there. I went for a walk and did some shopping. My dr advised me not to work out on the first day (not sure why) so I didn't (I also think it was a scheduled rest day for me). I still felt pretty bad from the oral progesterone so I didn't mind taking a rest day. I believe I went back to my normal training the next day. I know I did double century ride on the weekend following the insertion (which I think happened on a Wed). In general, the first couple of weeks, I had real minor cramping but not every day or constantly. Just something that periodically happened. The cramps really weren't painful but more of a slight discomfort - I sort of just felt off. Sometimes they were quick and a bit sharper like a quick jab to my stomach and other times it was just a soreness in the abdominal area for a couple of hours or so. But nothing too painful to keep me from training (I was training for IMWI at the time). I also had spotting from week 3-8 or so. I used liners as a preventive measure during this time. After 8 weeks from insertion, the cramps went away and the spotting has almost completely disappeared and I have not had a real period since. However, I do spot during hard workouts, especially long run but the spotting is very minimal. I also really haven't had many other side effects that are negative (that I can directly associate with the mirena).
Based people I have talked to and things I have read, the insertion can be as easy as mine was or alot more painful. I would just plan for the worst and make changes based on how you feel. You likely will be able to return to work but you may just want to be prepared in case you can't. Same with working out.
There is a community/message board for women with IUDs and women who want IUDs that I found helpful (see below). Most of the women here really love their IUDs and are up front and honest about their experiences.
http://community.livejournal.com/iud_divas/ Hope some of this helps. And feel free to ask me more specific questions if you have any.