Hoping we have some Central Park experts in fodors.
During part of my trip we will be going from St. Pat's (E 50th St x 5th/6th Ave) to Strawberry Fields Memorial (72nd St x Central Park West). The theory is to go through CP, which brings up a few questions: 1. is it too far / too warm to walk that distance Mid-August at midday (39 to 50 yr olds)? 2. if ok to walk - any suggestion on where to enter (6th Ave on South side or one of the 60's on the East side). 3. if NOT ok to walk the whole way, should we take a bus up Madison to E 72nd and walk straight across the middle?

It's a nice walk from the cathedral up to 59th street. If it's hot you could duck into Tiffanys (at 57th St.) to cool off and admire the merchandise. You can enter the park across from the Plaza Hotel and wend your way to 72nd Street. I've lived in NYC my entire adult life and I always am in wonder at Central Park when I decide to stroll thru. There should be a map of the park someplace to highlight interesting spots on your stroll. (If I find one.. I'll post it.)
Now..... if it's a real scorcher (sp?) of a day (and we can't predict those) the Mad Ave bus is a good option. There are bus stops right behind St. Patricks.

http://www.centralpark2000.com/produ.../map_home.html

Check out the above address... it's really thorough.

For a nice walk the entire way, enter the park at the corner of Fifth Avenue and 59th St. Walk north through the zoo -- you can pass through without paying the fee to enter the area of the exhibits. Just north of the zoo you can visit the Balto statue. Then I'd veer west to stop at the Dairy and carousel. From the carousel I'd head northeast to walk along the Sheep Meadow and reach the Mall (famous spot often appears in movies) and follow the Mall all the way to the Bethesda Terrace and Lake. From the terrace, it's a short walk west to Strawberry Fields.

Your idea to walk straight across the middle is a good option. At 72nd Street, you could head to the Conservatory Water, the toy sailboat pond, and then head to the Boathouse and walk along the Lake to the Bethesda Terrace.

The distance from St. Patrick's to Strawberry Fields in between a mile and quarter and mile and a half, which is certainly walkable. And, you can always find a shady side when walking up Fifth Avenue because of the tall buildings. I certainly wouldn't bother with a bus.
As a general guideline in New York, 20 north/south blocks equal one mile. East/west blocks are much longer and never more than 10 to a mile, and often even fewer depending where you are.

I would do the whole walk and I am older than you, but it depends on how used to walking all of you are and the humidity that day. If it is very humid (like today), you will be uncomfortable. I will suggest two walks that are some of my favorites. One is to enter on 5th across from the Plaza hotel and take Poet's Walk up to the 72nd St transverse, cross over and see Bethesda Fountain. Go back to 72nd St and walk east (toward 5th). Before you get to the 5th Ave exit, turn left and follow the path down to the Conservatory Water (the sailboat pond). Circle the pond, rest on a bench in the shade, get a cool snack or drink at the kiosk. Then pass the Alice in Wonderland statue and head back to 72nd St. At the transverse again, turn right and follow it west to Strawberry Fields. If that is too much, take a bus up Madison to 76th St. Walk over to 5th Ave and enter the park at 76th St. Follow the path that curves left (south). You will soon be at the Alice statue and the pond. Keep heading south, uphill after you pass the pond and it will take you to 72nd St (still in the park). Turn right and follow this road west to Strawberry Fields. I wouldn't consider this second walk long at all. This will make sense if you look at a map. But, and I say this seriously, many locals get lost in the park (my friends and dh included) - because the paths curve. Please ask anyone as you're walking if you're not sure that you're headed in the right directions.

I type slowly - just saw the above suggestions - very good options.

First of all, St Pat's faces onto the East side of Fifth Avenue (actually between fifth and Madison, not fifth and sixth).

The walk is certainly doable - but long - and I wouldn't walk that whole distance in very hot/humid weather. There's so much to explore in the Park that I would take a bus up Madison and then walk across the Park near 72 St.

If you go to the Central Park conservancy web site you can print out a map of the major paths in the park.

St. Patrick's cathedral is between Madison & Fifth NOT 5th & 6th. Maps.google.com will be a big help to you. I'd walk up Fifth ave. if you want to do some window shopping along Fifth ave. if it's not too humid. If it is too hot, take the Madison Ave. bus to 64 st. and enter on Fifth & 64-65 st. near the zoo.