+1 on heartrate. Use heartrate and time to compare, in a very broad sense, workload. Before I had a power meter I used a very basic HR monitor (last iteration was a Timex watch with HR, before that I had Polar HRM, and in there somewhere I had a cyclocomputer with HR). The Timex was nice, it gave me avg HR plus the amount of time spent in one zone. I set a zone that was basically "very hard" so I could track how much time I spent going hard. For me that was about 160 bpm, which is what I hit if I'm working pretty hard on the trainer, but which I generally average in a race.
We have an almost 11 month old here (March 10th birthday). I did a few outdoor rides over the summer but I've been doing almost all indoor rides since he was born, at least on non-race days. On the trainer I can instantly respond to him if something happens. The Missus doesn't feel abandoned (we work well together I guess - he's our first). We all go together to my races but I basically skipped doing any group rides. I feel better about spending time on the bike because I can respond to requests for help quickly. So for example if he blew through a diaper and it's all over his legs and such I can go help contain the mess. Or he is hungry and wet I can change him while she gets food or vice versa. Some of my trainer days were 12 mnutes here, 20 minutes there, etc. Inevitably if I get on the trainer thinking "man he was sooooo tired he's gonna sleep for 2 hours" he gets up in 12 minutes. If he doesn't seem tired then I don't get on the bike and that's when he sleeps for 2 or 3 hours.
You can see all my training in Strava:
http://app.strava.com/athletes/143064
I started logging everything there sometime in April so May 2012 to now is accurate. With different sleep schedules my riding changed. He used to fall asleep 9:30-10:15 PM and wake up at 6. Now he's falling asleep at 7-ish and waking up at 7-ish, sometimes with a wake-up at some point between 11 PM and 3 AM (last night was 11 PM). I can do longer rides in the evening as a result. The 5 hour and 3 hour weekend in early January Junior and the Missus were off visiting relatives so I had free time. I basically got on the bike Saturday when they left and climbed off on Sunday shortly before they got home (I wanted to do 5 hours Sunday but they got back earlier than I expected and I really needed to get some stuff done before they got back home).
A local ex-pro Cat 1 coach/racer (one of his guys signed for Jelly Belly for 2013) is a former high school runner. He did 4:30 miles apparently so he's a fast runner as well as a very strong racer. He said a good rule of thumb is to multiply running time by three to calculate recovery time. In other words an hour run is like riding three hours, in terms of recovery needed. This is not a training workload calculation, it's recovery time calculation.
I mention this because with running being so hard on the joints/muscles you may find a very light easy trainer ride on Monday helpful in getting the blood flowing. "Active recovery", like an easy day, might be better than absolute rest if your legs are really sore.
Likewise a longer base type ride on Saturday won't overly fatigue you for Sunday's long run but it'll still help with your cycling foundation/base.
Trainers have other benefits. You can experiment with fit, form, adjustments, etc without risking stranding or injuring yourself. As soon as something seems wrong you can stop. You won't have that "well I can just ride home" thing going. You'll have whatever tools you own close at hand. Etc. Usually after I build a bike I ride it on the trainer for a few hours before heading out. I hone bar and lever angles, saddle position, and double check all the adjustments. After a few hours of riding I'll wrap the bars - I'll have confirmed the proper bar angle and lever placement.
Hope this helps. Congrats on your newborn too.
cdr